Chemistry: Moles and Avogadro's Constant

Questions and Answers

What is the mass of oxygen in 30 g of Al2O3?

12 g

30 g

16 g

14.1 g (correct)

What is the relative atomic mass of Neon (Ne)?

20 (correct)

If the density of water at 4 oC is 1.00 g cm-3, what is the mass of 1 mole of H2O?

0.500 g

1.00 g

9.01 g

18.02 g (correct)

Which of the following represents a metal with a relative atomic mass of 56?

Iron (Fe) Signup and view all the answers

Relative molecular mass is used to define the relative mass of what type of substance?

One molecule of a covalent substance Signup and view all the answers

What is the mass of 1 mole of magnesium?

24g Signup and view all the answers

How many magnesium atoms are contained in 0.5 moles of magnesium?

3.01 x 10^23 atoms Signup and view all the answers

What does Avogadro's constant represent?

Number of particles in a mole Signup and view all the answers

How many hydrogen atoms are present in 1 mole of H2O?

2.408 x 10^24 Signup and view all the answers

If you have 2 moles of NH4Cl, how many hydrogen atoms are contained within?

4.816 x 10^23 H atoms Signup and view all the answers

How many ion particles are found in 0.20 mol of (NH4)3PO4?

4.8 x 10^23 Signup and view all the answers

What is the total number of particles in 0.100 mol of [Pt(NH3)2Cl2]?

6.62 x 10^23 Signup and view all the answers

Why does relative atomic mass not have any units?

It compares to a standard mass. Signup and view all the answers

What is the empirical formula of the molecular formula C2H6?

CH3 Signup and view all the answers

If a compound has 79.8 g of Carbon and 20.2 g of Hydrogen, what is its empirical formula?

C2H5 Signup and view all the answers

What is the first step in calculating the empirical formula from the formula mass?

Calculate the moles of each element present. Signup and view all the answers

To find the molecular formula from the empirical formula, what must be calculated first?

The empirical formula mass. Signup and view all the answers

If the empirical formula of a compound is CH2O and its molecular formula mass is 180.0 g, what is the molecular formula?

C6H12O6 Signup and view all the answers

In determining the empirical formula, why do you divide the moles of each element by the smallest number of moles present?

To simplify ratios to whole numbers. Signup and view all the answers

Given 2.199 g of Copper and 0.277 g of Oxygen, what is the empirical formula calculated?

Cu2O Signup and view all the answers

What common mistake might result in incorrect empirical formula subscripts?

Using decimal subscripts instead of whole numbers. Signup and view all the answers

What is the correct empirical formula derived from the percent composition of a compound with 38.7% Carbon, 51.6% Oxygen, and 9.7% Hydrogen?

CH3O Signup and view all the answers

If the molecular weight of a compound is 62 g/mol and its empirical formula has a molecular weight of 31 g/mol, what is the factor by which the empirical formula must be multiplied to obtain the molecular formula?

2 Signup and view all the answers

To find the empirical formula from percent composition, which step is performed first?

Convert grams to moles Signup and view all the answers

What must be true about the subscripts in the empirical formula?

They must be whole numbers. Signup and view all the answers

If a sample of a substance contains 20.2% Carbon, 11.4% Nitrogen, and 65.9% Oxygen, what is the first step in determining its empirical formula?

Convert grams to moles. Signup and view all the answers

What is the molar mass of CH4N?

30 g Signup and view all the answers

What is the empirical formula for a compound if the molecular formula is C2H6O2?

CH3O Signup and view all the answers

Which of the following compounds would likely have a higher molecular weight than CH4N?

C2H6O2 Signup and view all the answers

What is the molarity of hydrogen ions in a 0.15 molar solution of sulfuric acid?

0.3 M Signup and view all the answers

How many moles of solute are in 40.0 cm3 of a 0.500 mol/dm3 solution?

0.0200 mol Signup and view all the answers

What mass of iron (II) sulfate is present in 100 cm3 of a 0.1 mol dm-3 solution?

1.52 g Signup and view all the answers

Given a solution with 0.0250 mol of solute and a total volume of 25.0 cm3, what is its molar concentration?

1.00 mol/dm3 Signup and view all the answers

How many cm3 of a 1.00 mol dm-3 solution contains 2.50 x 10-3 mol of solute?

2.50 cm3 Signup and view all the answers

What is the molarity of a solution formed from 1 mole of a substance in 1 liter of solution?

1 mol/dm3 Signup and view all the answers

Which equation represents the dissociation of sulfuric acid in solution?

H2SO4 ⇆ 2H+ + SO4^2- Signup and view all the answers

What is the formula to calculate the mass of solute in g/dm3?

mol/dm3 x molar mass Signup and view all the answers

Study Notes

Amounts in Chemistry

One mole of any substance has a mass equivalent to its relative atomic mass in grams.
Example: 1 mole of magnesium (Mg) weighs 24g and contains approximately 6.02 x 10²³ atoms.
12g of magnesium corresponds to 0.5 moles, containing 3.01 x 10²³ magnesium atoms.

Avogadro's Constant

Avogadro's constant (Na) is 6.02 x 10²³ mol⁻¹, indicating the number of particles in one mole.
This constant applies to all types of particles: atoms, molecules, ions, protons, neutrons, or electrons.

Quick Calculations with Moles

For various amounts of moles:
- 1 mol contains 6.02 x 10²³ atoms.
- 2 mol contains 1.204 x 10²⁴ atoms.
- 10 mol contains 6.02 x 10²⁴ atoms.
- 0.1 mol contains 6.02 x 10²² atoms.

Counting Atoms and Ions in Compounds

Calculating hydrogen atoms in compounds:
- In 2 mol of H₂O, there are 2.408 x 10²⁴ hydrogen atoms.
Sample calculations include:
- 0.5 mol H₂O results in 6.02 x 10²³ hydrogen atoms.
- 0.2 mol NH₄Cl results in 4.816 x 10²³ hydrogen atoms.
- 1 mol Al(HCO₃)₃ results in 1.806 x 10²⁴ hydrogen atoms.

Relative Atomic Mass and Formula Mass

Relative atomic mass is unitless and calculated without specific units.
Finding the mass of an element in compounds, e.g., 30g of Al₂O₃ contains 14.1g oxygen.

Molar Mass

Molar mass defines how much one mole of a substance weighs.
Example: 0.500 mol of H₂O at 4°C occupies a volume of 9.01 cm³ and has a density of 1.00 g/cm³.

Empirical Formulas

The empirical formula represents the simplest whole-number ratio of elements in a compound.
Derived by calculating the moles of each element, dividing by the smallest number of moles, and obtaining whole numbers.
Example: A compound with 2.199g Cu and 0.277g O has an empirical formula of Cu₂O.

Finding Molecular Formulas

The molecular formula is a whole-number multiple of the empirical formula.
To determine it, calculate the formula mass, divide the given mass by this value, and adjust the empirical formula accordingly.
Example: For a compound with a molar mass of 60.0g and an empirical formula of CH₄N, the molecular formula calculates to C₂H₈N₂.

Molar Concentrations (Molarity)

Molarity (M) is the concentration of a solution and calculated as moles of solute per liter of solution (mol/dm³).
Example: A 0.15 molar solution of H₂SO₄ provides 0.30 mol/dm³ of hydrogen ions upon dissociation.
Mass required for a specific molarity can be calculated using: moles x relative formula mass.

Practical Calculations

Calculate mass or volume needed for a specific molar concentration.
Example: To prepare 0.1 mol/dm³ of sulfuric acid, calculate needed mass based on volume and molarity requirements.

Tips for Empirical/Molecular Formula Calculations

Start with percent compositions and convert to grams to find moles.
Always check that subscripts in formulas are whole numbers.
Use sample weights to make calculations simpler when handling percentages.

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Description

This quiz covers the fundamentals of moles in chemistry, including the concept of Avogadro's constant and how to calculate the number of particles in a given amount of substance. It features examples and calculations to help reinforce understanding of these essential topics.