Density Calculations and Lab Practices

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Questions and Answers

What is the calculated density of the irregularly shaped object?

  • 1.77 g/mL
  • 1.44 g/mL (correct)
  • 0.93 g/mL
  • 2.35 g/mL

What is the student's percent error when comparing the calculated density to the accepted density?

  • 20.4%
  • 12.5%
  • 5.2%
  • 18.6% (correct)

Why is a graduated cylinder preferred over a beaker for measuring liquid volume?

  • Graduated cylinders provide more significant figures. (correct)
  • Graduated cylinders have a larger volume.
  • Graduated cylinders are less fragile.
  • Graduated cylinders are more diverse in shapes.

Which of the following accurately describes a hypothesis?

<p>A predictive if-then statement. (A)</p> Signup and view all the answers

In the context of an experiment, what is an independent variable?

<p>A variable that is altered. (C)</p> Signup and view all the answers

What is the primary use of an Erlenmeyer flask?

<p>To hold liquids and allow for swirling. (C)</p> Signup and view all the answers

How many significant figures are reported in the measurement of 4.84 cm?

<p>3 significant figures (D)</p> Signup and view all the answers

What unit is most appropriate for measuring the mass of a pencil?

<p>g (C)</p> Signup and view all the answers

Which individual is more precise in their measurements of the melting point of ice?

<p>Student A (C)</p> Signup and view all the answers

What significant conclusion did Rutherford conclude from his gold foil experiment?

<p>Atoms are mostly empty space with a dense nucleus. (B)</p> Signup and view all the answers

How many protons does sulfur-36 contain?

<p>16 (A)</p> Signup and view all the answers

What is the formula for the compound formed between potassium and iodine?

<p>KI (B)</p> Signup and view all the answers

Which group of elements contains those with the same number of valence electrons?

<p>Elements in the same group (D)</p> Signup and view all the answers

What differentiates carbon-14 from carbon-12 and carbon-13?

<p>It is a radioactive isotope. (B)</p> Signup and view all the answers

What is the charge of neutrons in an atom?

<p>No charge (D)</p> Signup and view all the answers

In which group do alkali metals reside in the periodic table?

<p>Group 1 (D)</p> Signup and view all the answers

Which type of bond would barium and sulfur form?

<p>Ionic bond (B)</p> Signup and view all the answers

Which subatomic particle has the least mass?

<p>Electron (D)</p> Signup and view all the answers

Which of these groups contain elements that typically do not form chemical bonds?

<p>Noble gases (A)</p> Signup and view all the answers

How is the modern periodic table organized?

<p>By atomic number (D)</p> Signup and view all the answers

What happens to an atom's potential energy when it forms full chemical bonds?

<p>Decreases (B)</p> Signup and view all the answers

What is an example of a chemical change?

<p>Burning wood (D)</p> Signup and view all the answers

What characterizes a physical change compared to a chemical change?

<p>It alters physical properties without changing composition. (B)</p> Signup and view all the answers

Which of the following is an example of nuclear fission?

<p>Splitting of a uranium nucleus (C)</p> Signup and view all the answers

What occurs during an exothermic reaction?

<p>More energy is released than absorbed. (B)</p> Signup and view all the answers

Which statement is true regarding the molecular structure of NH3?

<p>Nitrogen and hydrogen form single covalent bonds. (D)</p> Signup and view all the answers

What is produced during the combustion of propane (C3H8)?

<p>CO2 and H2O (C)</p> Signup and view all the answers

What is the molar mass of CH2F2?

<p>52.03 g/mol (B)</p> Signup and view all the answers

Which reaction type is exemplified by the equation Al(NO3)3 + 3NaOH -> Al(OH)3 + 3NaNO3?

<p>Double replacement (A)</p> Signup and view all the answers

What happens to molecules during boiling water?

<p>Intermolecular bonds are broken. (D)</p> Signup and view all the answers

How many moles of H2 are produced if 2 moles of CH4 are reacted?

<p>8 moles (B)</p> Signup and view all the answers

Which of the following bonds involves the equal sharing of electrons?

<p>Nonpolar covalent bond (B)</p> Signup and view all the answers

What is the primary energy conversion in photosynthesis?

<p>Light to chemical potential energy (C)</p> Signup and view all the answers

Which type of reaction involves carbon and oxygen forming CO2?

<p>Synthesis reaction (A)</p> Signup and view all the answers

What type of bonds are formed between carbon and oxygen in CO2?

<p>Double bonds sharing four electrons (B)</p> Signup and view all the answers

Which of the following describes a nuclear decay reaction?

<p>Emission of particles from an unstable nucleus (C)</p> Signup and view all the answers

Flashcards

Density

The ratio of an object's mass to its volume.

Percent error

The difference between experimental and accepted values, expressed as a percentage of the accepted value.

Graduated cylinder vs. beaker

A tool with more graduations (lines) provides more precise measurements and therefore more significant figures.

Hypothesis

A testable prediction in the form of an ‘if, then, because’ statement.

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Independent variable

The variable that is changed or manipulated in an experiment.

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Dependent variable

The variable that is measured in an experiment to observe the effect of the independent variable.

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Control

A standard for comparison in an experiment.

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Constant

Factors that are kept constant throughout an experiment.

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Precision in measurement

The closeness of repeated measurements to each other., A measurement is precise if it is consistent.

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Accuracy in measurement

How close a measurement is to the true or accepted value.

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Scientific Notation

A way to express very large or very small numbers in a compact form. It consists of a number between 1 and 10 multiplied by a power of 10.

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Standard Form

The process of converting a number from scientific notation to standard form.

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Atomic Nucleus

A dense, positively charged region at the center of an atom, consisting of protons and neutrons.

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Protons

Positively charged particles found in the nucleus of an atom. Their number determines the element's identity.

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Neutrons

Neutral particles found in the nucleus of an atom. Their number can vary creating isotopes.

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Electrons

Negatively charged particles that orbit the nucleus of an atom in specific energy levels.

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Isotopes

Atoms of the same element that have the same number of protons but a different number of neutrons.

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Atomic Number

The number of protons in the nucleus of an atom. It defines the element.

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Elements in the same group

Atoms of elements in the same vertical column (group) of the periodic table. They share the same number of valence electrons and therefore similar chemical properties.

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Elements in the same period

Atoms of elements in the same horizontal row (period) of the periodic table. They have their valence electrons on the same energy level.

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Covalent Bonds

A strong bond formed by the sharing of valence electrons between atoms. These bonds are very stable and usually found in non-metals.

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Ionic Bonds

A bond formed by the transfer of electrons from a metal to a non-metal. These bonds create ions.

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Intermolecular Forces

Interactions between different molecules. They determine the physical properties of a substance like melting point.

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Physical Change

A change that alters the physical properties of a substance without changing its chemical composition. Examples include melting ice, boiling water, or dissolving sugar in water.

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Chemical Change

A change that alters the chemical composition of a substance, resulting in the formation of new substances with different properties. Examples include burning wood, rusting iron, or cooking an egg.

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Nuclear Reaction

A change within an atom's nucleus, involving the emission of particles or the combination or splitting of nuclei. Examples include nuclear decay, fission, and fusion.

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Chemical Reaction

A change involving the rearrangement of atoms and molecules to form new substances. It does not involve changes to the nucleus of atoms.

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Nuclear Decay

A process where an unstable nucleus releases energy and particles to become more stable. Examples include the decay of uranium-235 to lead-207.

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Nuclear Fission

A process where a large, unstable nucleus splits into two or more smaller nuclei, releasing a tremendous amount of energy. Examples include the splitting of uranium-235 in nuclear power plants.

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Nuclear Fusion

A process where two or more small nuclei combine to form a larger, more stable nucleus, releasing a tremendous amount of energy. Examples include the fusion of hydrogen isotopes in the sun.

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Energy Changes in Chemical Reactions

Energy must be absorbed to break bonds in reactants. As new bonds form in products, energy is released.

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Exothermic Reaction

A chemical reaction that releases more energy than it absorbs. The change in enthalpy (DHrxn) is negative.

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Endothermic Reaction

A chemical reaction that absorbs more energy than it releases. The change in enthalpy (DHrxn) is positive.

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Intermolecular Bonds

Forces of attraction that act between molecules. They influence a substance's melting point, boiling point, and viscosity.

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Intramolecular Bonds

Forces of attraction that act within a molecule, holding atoms together. They determine the shape and structure of a molecule.

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Photosynthesis

The process of converting light energy from the sun into chemical energy stored in glucose molecules. It is an endothermic process.

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Cellular Respiration

The process of breaking down glucose molecules to release energy for cellular processes. It is an exothermic process.

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Combustion

A chemical reaction that releases energy, usually in the form of heat and light. It is commonly associated with burning fuels. It is an exothermic process.

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Nuclear Fission

A process that harnesses energy from the splitting of large, unstable nuclei into smaller, more stable nuclei. It is an exothermic process.

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Study Notes

Density Calculations and Errors

  • Density formula: Density = Mass / Volume
  • Object's mass: 103 g
  • Initial water volume: 22.4 mL
  • Final water volume: 93.8 mL
  • Volume of object: 93.8 mL - 22.4 mL = 71.4 mL
  • Calculated density: 103 g / 71.4 mL = 1.44 g/mL
  • Accepted density: 1.77 g/mL
  • Percent error: |1.44 - 1.77| / 1.77 * 100% = 18.6%

Graduated Cylinder vs. Beaker

  • Graduated cylinder: Provides more precision in measurements due to more graduations. More graduations leads to more significant figures in the measurement, improving the expressed uncertainty.
  • Important practices using a graduated cylinder:
    • Read the measurement at eye level.
    • Read from the bottom of the meniscus (the curved surface of the liquid).
    • Place the cylinder on a flat surface.

Hypothesis, Variables, and Controls

  • Hypothesis: A "if...then...because" statement predicting the experiment's outcome. (e.g., If parachute size increases, then fall time increases because increased size increases air resistance.)
  • Independent variable: The variable changed in the experiment. (e.g., parachute size)
  • Dependent variable: The variable measured to see the independent variable's effect. (e.g., fall time)
  • Control: The condition for comparison. (e.g., none in parachute experiment).
  • Constants: Things kept the same between trials. (e.g., parachute material, parachute shape)

Erlenmeyer Flask

  • Shape: Sloped sides.
  • Use: Holding liquids, good for swirling.

Ruler Measurements

  • Ruler A: 4.8 cm (2 significant figures)
  • Ruler B: 4.84 cm (3 significant figures)

Measurement Units and Tools

  • Mass of pencil: grams (g), electronic balance
  • Length of desk: centimeters (cm), ruler or meter stick
  • Temperature of coffee: degrees Celsius (°C), thermometer or temperature probe

Melting Point Precision and Accuracy

  • Student A: More precise (measurements close together). More accurate (measurements closer to accepted value).
  • Student B: Less precise (wider range of measurements). Less accurate.

Scientific Notation

  • 0.00443: 4.43 x 10-3
  • 35889: 3.5889 x 104

Standard Form

  • 3.71 x 10-3: 0.00371
  • 8.825 x 105: 882500

Atomic Structure Discoveries

  • Bohr: Observed emission spectra, electrons orbit nucleus in specific energy levels (further orbits = higher energy).
  • Rutherford: Gold foil experiment, discovered a dense, positively charged nucleus in the atom.
  • Thomson: Cathode ray experiment, discovered the electron (negatively charged).

Periodic Table

  • Mendeleev: Created the first periodic table, ordered by atomic mass.
  • Modern periodic table: Ordered by atomic number (number of protons).

Periodic Table Groups and Properties

  • Same group: Same number of valence electrons, similar chemical properties.
  • Same period: Valence electrons on the same shell/energy level.

Subatomic Particles

  • Proton: Positive charge, ~1 amu, located in the nucleus.
  • Neutron: No charge, ~1 amu, located in the nucleus.
  • Electron: Negative charge, ~1/2000 amu, located in orbitals.

Isotopes and Nuclear Notation

  • Sodium-24: 24Na11 (protons=11, neutrons = 13, electrons= 11)
  • Sulfur-36: 36S16 (protons=16, neutrons=20, electrons=16)

Isotopes of Carbon

  • Carbon-12, Carbon-13, Carbon-14: Different numbers of neutrons.
  • Radioactivity (Carbon-14): Unstable nucleus due to neutron number.

Valence Electrons

  • Groups 1-2: 1-2 valence electrons
  • Groups 13-18: 3-8 valence electrons (Helium has 2).

Atomic Stability

  • Stability: Full valence shell (octet) by forming chemical bonds.
  • Part involved: Valence electrons.
  • Elements without bonds: Noble gases (full octet).

Ions

  • K: K+ (loses 1 electron)
  • Be: Be2+ (loses 2 electrons)
  • Br: Br− (gains 1 electron)
  • Al: Al3+ (loses 3 electrons)

Potassium and Iodine Compound

  • Formula: KI (potassium iodide)
  • Reason for electron loss: Potassium (metal) loses electrons in an ionic bond.

Barium and Sulfur Compound

  • Bond type: Ionic bond (metal and nonmetal).
  • Formula: BaS (barium sulfide).

Compound Identification

  • a. Low melting point, non-conducting: Covalent compound
  • b. Ductile, conducting: Metal
  • c. High melting point, dissolves in water: Ionic compound

Intermolecular Bonding and Melting

  • Stronger intermolecular bonds: Higher melting point.

Intermolecular Bonds (Increasing Strength)

  • London dispersion forces
  • Dipole-dipole forces
  • Hydrogen bonding
  • Ionic bonds

Physical vs. Chemical Changes

  • Physical: Alters physical properties but not composition (e.g., melting ice).
  • Chemical: Alters chemical composition (e.g., rusting).
  • Indicators of chemical change: Color change, odor change, light, temperature change, precipitate formation, gas formation.

Lewis Structures of NH3 and CF4

  • NH3: Nitrogen forms single bonds to 3 Hydrogens.
  • CF4: Carbon forms single bonds to 4 Fluorines. Structures illustrate fulfilling octet rule for each atom.

Nuclear vs. Chemical Reactions

  • Nuclear: Nucleus changes (e.g., radioactive decay, fission, fusion).
  • Chemical: Arrangement of atoms changes (e.g., combustion).

Balanced Chemical Equations

  • a. C3H8 + 5O2 → 3CO2 + 4H2O: Combustion
  • b. 2NH3 → N2 + 3H2: Decomposition
  • c. Al(NO3)3 + 3NaOH → Al(OH)3 + 3NaNO3: Double replacement
  • d. MnO2 + 2Mg → 2MgO + Mn: Single replacement
  • e. 2Al + 3Cl2 → 2AlCl3: Synthesis

Energy Changes in Reactions

  • Negative ΔHrxn: Exothermic (more energy released forming than absorbed breaking bonds).
  • Positive ΔHrxn: Endothermic (more energy absorbed breaking than released forming bonds).

Chemical Equation Analysis (CH4 + 2H2O → CO2 + 4H2)

  • a. [CH4] + 2H2O → [CO2] + 4H2
  • b. ΔHrxn = 162 kJ (calculations shown).
  • c. Endothermic.
  • d. Polar covalent bond, 4 electrons shared.
  • e. 8 moles of H2.
  • f. Nonpolar covalent bond, 2 electrons shared equally.

Mole Calculations (CH2F2)

  • a. Molar mass: 52.03 g/mol
  • b. Mass of 4.66 mol: 242.46 g
  • c. Number of molecules in 17.6 g: 2.04 x 1022 molecules.

Intermolecular vs. Intramolecular Bonds

  • Intramolecular: Bonds within a molecule holding atoms together.
  • Intermolecular: Forces between molecules.
  • Boiling water: Breaks intermolecular bonds.

Processes as Endothermic or Exothermic

  • a. Photosynthesis: Endothermic (light energy to chemical energy).
  • b. Cell respiration: Exothermic (chemical to heat, kinetic, potential).
  • c. Combustion: Exothermic (chemical to heat and kinetic energy).

Coal vs. Nuclear Power

  • Coal: Chemical reaction (combustion), releases energy by breaking chemical bonds, produces pollutants.
  • Nuclear: Nuclear change (fission), releases energy by changing the nucleus of an atom, produces radioactive waste.

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