Q: What is Avogadro's number and what does it represent?

$N_A = 6.022 \times 10^{23}$; the number of particles (atoms, molecules, etc.) in one mole.

Q: Calculate the formula mass of $C_{12}H_{22}O_{11}$ (sucrose) in amu using the following atomic masses: C = 12.01 amu, H = 1.008 amu, O = 16.00 amu.

$(12 \times 12.01) + (22 \times 1.008) + (11 \times 16.00) = 342.30$ amu

Q: A sample contains 4.50 moles of Al. What is the mass of Al in grams? (Atomic mass of Al = 26.98 g/mol)

$4.50 \text{ mol} \times 26.98 \text{ g/mol} = 121.4 \text{ g}$

Q: How many $NH_3$ molecules are present in 45 g of $NH_3$? (Molar mass of $NH_3$ = 17.03 g/mol)

$45 \text{ g} \times \frac{1 \text{ mol}}{17.03 \text{ g}} \times 6.022 \times 10^{23} = 1.6 \times 10^{24}$ molecules

Q: How many oxygen atoms are present in 14.3 g of $Cr_2(SO_4)_3$? (Molar mass = 392.17 g/mol)

$14.3 \text{ g} \times \frac{1 \text{ mol}}{392.17 \text{ g}} \times \frac{12 \text{ mol O}}{1 \text{ mol}} \times 6.022 \times 10^{23} = 2.64 \times 10^{23}$ atoms

Q: What is the mass percent composition of hydrogen in $H_2O$?

$\frac{2 \times 1.008}{18.02} \times 100\% = 11.19\%$

Q: A 4.30 g sample of butyric acid produces 8.59 g of $CO_2$ and 3.52 g of $H_2O$ upon combustion. What is the mass percent of carbon in butyric acid?

$\frac{2.34}{4.30} \times 100\% = 54.4\%$

Question 1

Which statement best describes the mole in chemistry?

Accepted Answer

It is the SI unit for amount of substance, connecting molecular and lab scales.

Question 2

What is Avogadro's number and what does it represent?

Accepted Answer

$N_A = 6.022 	imes 10^{23}$; the number of particles (atoms, molecules, etc.) in one mole.

Question 3

Calculate the formula mass of $C_{12}H_{22}O_{11}$ (sucrose) in amu using the following atomic masses: C = 12.01 amu, H = 1.008 amu, O = 16.00 amu.

Accepted Answer

$(12 	imes 12.01) + (22 	imes 1.008) + (11 	imes 16.00) = 342.30$ amu

Question 4

How are formula mass and molar mass related?

Accepted Answer

Formula mass and molar mass are numerically equal, but formula mass is in amu and molar mass is in g/mol.

Question 5

A sample contains 4.50 moles of Al. What is the mass of Al in grams? (Atomic mass of Al = 26.98 g/mol)

Accepted Answer

$4.50 	ext{ mol} 	imes 26.98 	ext{ g/mol} = 121.4 	ext{ g}$

Question 6

How many $NH_3$ molecules are present in 45 g of $NH_3$? (Molar mass of $NH_3$ = 17.03 g/mol)

Accepted Answer

$45 	ext{ g} 	imes \frac{1 	ext{ mol}}{17.03 	ext{ g}} 	imes 6.022 	imes 10^{23} = 1.6 	imes 10^{24}$ molecules

Question 7

How many oxygen atoms are present in 14.3 g of $Cr_2(SO_4)_3$? (Molar mass = 392.17 g/mol)

Accepted Answer

$14.3 	ext{ g} 	imes \frac{1 	ext{ mol}}{392.17 	ext{ g}} 	imes \frac{12 	ext{ mol O}}{1 	ext{ mol}} 	imes 6.022 	imes 10^{23} = 2.64 	imes 10^{23}$ atoms

Question 8

What is the mass percent composition of hydrogen in $H_2O$?

Accepted Answer

$\frac{2 	imes 1.008}{18.02} 	imes 100\% = 11.19\%$

Question 9

An organic compound containing C, H, and O undergoes combustion, producing only $CO_2$ and $H_2O$. How is the mass percent of oxygen in the compound determined?

Accepted Answer

By subtracting the mass percents of C and H from 100%.

Question 10

A 4.30 g sample of butyric acid produces 8.59 g of $CO_2$ and 3.52 g of $H_2O$ upon combustion. What is the mass percent of carbon in butyric acid?

Accepted Answer

$\frac{2.34}{4.30} 	imes 100\% = 54.4\%$

Question 11

Which statement correctly distinguishes empirical and molecular formulas?

Accepted Answer

Empirical formula gives the simplest whole number ratio; molecular formula gives the exact number of each atom.

Question 12

Given the mass percent composition of adipic acid: 49.3% C, 6.9% H, and 43.8% O, what is the empirical formula?

Accepted Answer

$C_3H_5O_2$

Question 13

If the empirical formula of adipic acid is $C_3H_5O_2$ and its molecular mass is 146 amu, what is its molecular formula?

Accepted Answer

$C_6H_{10}O_4$

Question 14

Which of the following best describes a solution in general chemistry?

Accepted Answer

A homogenous mixture consisting of a solute dissolved in a solvent.

Question 15

What is the molarity of a solution containing 5.55 g of $CaCl_2$ in 125 mL of solution? (Molar mass of $CaCl_2$ = 110.98 g/mol)

Accepted Answer

$\frac{5.55 	ext{ g}}{110.98 	ext{ g/mol}} \div 0.125 	ext{ L} = 0.400 	ext{ M}$

Question 16

What volume (in mL) of 0.120 M $CuSO_4$ contains 1.50 g of $CuSO_4$? (Molar mass = 159.62 g/mol)

Accepted Answer

$\frac{1.50 	ext{ g}}{159.62 	ext{ g/mol}} \div 0.120 	ext{ mol/L} 	imes 1000 	ext{ mL} = 78.3 	ext{ mL}$

Question 17

A laboratory technician needs to prepare 500 mL of 3.75 M $HNO_3$ from a stock solution of 15.9 M $HNO_3$. What volume of stock solution is required?

Accepted Answer

$V_1 = \frac{M_2V_2}{M_1} = \frac{3.75 	imes 500}{15.9} = 118 	ext{ mL}$

Moles, Molar Mass, Empirical Formulas, and Solution Concentrations in General Chemistry

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