Q: Which formula is used to calculate the empirical formula mass?

$\text{Empirical formula mass} = \sum \text{atomic masses of all atoms in the empirical formula}$

Question 1

Which of the following best describes the molecular formula of a compound?

Accepted Answer

It gives the number of different elements in a compound.

Question 2

Given the empirical formula $\mathrm{CH_2O}$ and a molar mass of $180\ \mathrm{g/mol}$, what is the molecular formula?

Accepted Answer

$\mathrm{C_6H_{12}O_6}$

Question 3

A compound has an empirical formula of $\mathrm{C_2H_5}$ and a molar mass of $58\ \mathrm{g/mol}$. What is its molecular formula?

Accepted Answer

$\mathrm{C_4H_{10}}$

Question 4

Which step is necessary to determine the molecular formula from the empirical formula and molar mass?

Accepted Answer

Divide the molar mass by the empirical formula mass to find the n-factor.

Question 5

A compound contains 40% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. If its molar mass is $180\ \mathrm{g/mol}$, what is its molecular formula?

Accepted Answer

$\mathrm{C_6H_{12}O_6}$

Question 6

If the empirical formula mass of a compound is $58\ \mathrm{g/mol}$ and its molar mass is $174\ \mathrm{g/mol}$, what is the n-factor?

Accepted Answer

$n = 3$

Question 7

A sample of a compound contains 72.2% C, 8.5% H, and the remainder is O. If $0.250$ moles of the compound weighs $41.5\ \mathrm{g}$, what is the molar mass?

Accepted Answer

$166\ \mathrm{g/mol}$

Question 8

Which formula is used to calculate the empirical formula mass?

Accepted Answer

$	ext{Empirical formula mass} = \sum 	ext{atomic masses of all atoms in the empirical formula}$

Question 9

A compound has an empirical formula of $\mathrm{NPCl_2}$ and a molar mass of $347.64\ \mathrm{g/mol}$. What is its molecular formula?

Accepted Answer

$\mathrm{N_2P_2Cl_4}$

Question 10

A steroid hormone has a molar mass of $362.47\ \mathrm{g/mol}$ and contains 80.0% C, 8.5% H, and 11.5% O by mass. What is its molecular formula?

Accepted Answer

$\mathrm{C_{21}H_{30}O_2}$

Question 11

Why is it important to determine the n-factor when calculating the molecular formula?

Accepted Answer

It tells you how many times to multiply the empirical formula to get the molecular formula.

Question 12

Which of the following is the correct sequence of steps to determine the molecular formula from percent composition and molar mass?

Accepted Answer

Find moles of each element, determine empirical formula, calculate empirical formula mass, divide molar mass by empirical formula mass, multiply subscripts by n-factor.

Question 13

A compound has an empirical formula of $\mathrm{CH_2}$ and a molar mass of $56\ \mathrm{g/mol}$. What is its molecular formula?

Accepted Answer

$\mathrm{C_4H_8}$

Question 14

If a compound has a molar mass of $120\ \mathrm{g/mol}$ and an empirical formula mass of $60\ \mathrm{g/mol}$, what is the n-factor and the molecular formula if the empirical formula is $\mathrm{C_2H_4O}$?

Accepted Answer

n-factor is 2; molecular formula is $\mathrm{C_4H_8O_2}$

Question 15

Which of the following is NOT a correct reason for using empirical formulas in chemistry?

Accepted Answer

Empirical formulas provide information about the actual number of atoms in a molecule.

Molecular Formula: Calculation and Practice in Chemistry

Study Guide - Practice Questions

Study Guide - Flashcards