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?

A predictive if-then statement. Signup and view all the answers

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

A variable that is altered. Signup and view all the answers

What is the primary use of an Erlenmeyer flask?

To hold liquids and allow for swirling. Signup and view all the answers

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

3 significant figures Signup and view all the answers

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

g Signup and view all the answers

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

Student A Signup and view all the answers

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

Atoms are mostly empty space with a dense nucleus. Signup and view all the answers

How many protons does sulfur-36 contain?

16 Signup and view all the answers

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

KI Signup and view all the answers

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

Elements in the same group Signup and view all the answers

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

It is a radioactive isotope. Signup and view all the answers

What is the charge of neutrons in an atom?

No charge Signup and view all the answers

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

Group 1 Signup and view all the answers

Which type of bond would barium and sulfur form?

Ionic bond Signup and view all the answers

Which subatomic particle has the least mass?

Electron Signup and view all the answers

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

Noble gases Signup and view all the answers

How is the modern periodic table organized?

By atomic number Signup and view all the answers

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

Decreases Signup and view all the answers

What is an example of a chemical change?

Burning wood Signup and view all the answers

What characterizes a physical change compared to a chemical change?

It alters physical properties without changing composition. Signup and view all the answers

Which of the following is an example of nuclear fission?

Splitting of a uranium nucleus Signup and view all the answers

What occurs during an exothermic reaction?

More energy is released than absorbed. Signup and view all the answers

Which statement is true regarding the molecular structure of NH3?

Nitrogen and hydrogen form single covalent bonds. Signup and view all the answers

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

CO2 and H2O Signup and view all the answers

What is the molar mass of CH2F2?

52.03 g/mol Signup and view all the answers

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

Double replacement Signup and view all the answers

What happens to molecules during boiling water?

Intermolecular bonds are broken. Signup and view all the answers

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

8 moles Signup and view all the answers

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

Nonpolar covalent bond Signup and view all the answers

What is the primary energy conversion in photosynthesis?

Light to chemical potential energy Signup and view all the answers

Which type of reaction involves carbon and oxygen forming CO2?

Synthesis reaction Signup and view all the answers

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

Double bonds sharing four electrons Signup and view all the answers

Which of the following describes a nuclear decay reaction?

Emission of particles from an unstable nucleus Signup and view all the answers

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 ΔH_rxn: Exothermic (more energy released forming than absorbed breaking bonds).
Positive ΔH_rxn: Endothermic (more energy absorbed breaking than released forming bonds).

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

a. [CH₄] + 2H₂O → [CO₂] + 4H₂
b. ΔH_rxn = 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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Description

Test your knowledge on density calculations and laboratory measurement techniques in this quiz. Explore concepts such as percent error, the importance of using graduated cylinders, and the formulation of hypotheses in experiments. Perfect for students preparing for science assessments.