$\frac{\text{mass of element}}{\text{molar mass of compound}} \times 100$

$\text{Number of particles} = n \times N_A$

Question 1

Which of the following best describes the empirical formula for glucose ($C_6H_{12}O_6$)?

Accepted Answer

$CH_2O$

Question 2

If you have 32 g of $CuO$ and the molar mass of $CuO$ is 80 g/mol, how many moles of $CuO$ do you have?

Accepted Answer

$0.40$ mol

Question 3

What is the percentage by mass of oxygen in $CuO$? (Molar mass of $CuO = 80$ g/mol, mass of O = 16 g/mol)

Accepted Answer

$40	ext{%}$

Question 4

Given the formula $C_6H_{12}O_6$, what is the molecular formula mass?

Accepted Answer

$180$ g/mol

Question 5

Which equation correctly relates the number of moles ($n$), mass ($m$), and molar mass ($M$)?

Accepted Answer

$n = \frac{m}{M}$

Question 6

How many molecules are present in 2 moles of $H_2O$? (Use $N_A = 6.02 	imes 10^{23}$)

Accepted Answer

$1.20 	imes 10^{24}$

Question 7

A compound contains 40.1% oxygen by mass. If the total mass of the compound is 156 g, what is the mass of oxygen present?

Accepted Answer

$62.6$ g

Question 8

Which of the following is the correct formula for calculating percent composition by mass?

Accepted Answer

$\frac{	ext{mass of element}}{	ext{molar mass of compound}} 	imes 100$

Question 9

If a sample contains $1.25$ moles of $O_2$, how many molecules of $O_2$ are present?

Accepted Answer

$7.53 	imes 10^{23}$

Question 10

A compound has an empirical formula of $CH_2O$ and a molar mass of $180$ g/mol. What is its molecular formula?

Accepted Answer

$C_6H_{12}O_6$

Question 11

Which of the following best describes Avogadro's number?

Accepted Answer

The number of atoms in one mole of a substance

Question 12

If you have 64 g of $O_2$ (molar mass = 32 g/mol), how many moles of $O_2$ do you have?

Accepted Answer

$2$ mol

Question 13

Which of the following is NOT a step in determining the empirical formula from percent composition?

Accepted Answer

Multiply each percent by Avogadro's number

Question 14

A sample contains 20% sulfur by mass. If the total mass is 156 g, what is the mass of sulfur?

Accepted Answer

$31.2$ g

Question 15

Which of the following is the correct relationship for calculating the number of particles in a given number of moles?

Accepted Answer

$	ext{Number of particles} = n 	imes N_A$

Question 16

If the empirical formula of a compound is $CH_2O$ and its molar mass is 90 g/mol, what is the molecular formula?

Accepted Answer

$C_3H_6O_3$

Question 17

What is the mass percent of carbon in glucose ($C_6H_{12}O_6$)? (Atomic masses: C = 12, H = 1, O = 16)

Accepted Answer

$40	ext{%}$

Question 18

Which of the following is the correct formula for calculating the mass of a substance from moles and molar mass?

Accepted Answer

$m = n 	imes M$

Empirical and Molecular Formulas, Percent Composition, and Moles in General Chemistry

Study Guide - Practice Questions