Q: Calculate the formula mass of calcium hydroxide, Ca(OH)2, to three significant figures. (Atomic masses: Ca = 40.08, O = 16.00, H = 1.008)

$$40.08 + 2 \times (16.00 + 1.008) = 74.096$$ amu

Q: A sample contains $$0.253$$ mol of HNO$$_3. $$What is the mass in grams of this sample? (Molar mass of HNO$$_3$$ = $$63.02 g/$$mol)

$$0.253 \times 63.02 = 15.94 g$$

Q: The daily requirement of chromium in the human diet is $$1.0 \times 10$$^{-6}$$ g. How many atoms of chromium does this represent? (Atomic mass of Cr = 51.996$ g/mol, Avogadro's number N_A = 6.022$$ \times $$10^{23})$$

$$\frac{1.0 \times 10$$^{-6}$$}{51.996} \times 6.022 \times 10$$^{23}$$ = 1.16 \times 10$$^{16}$$ atoms

Q: What is the percentage composition of lead in PbCrO$$_4$$? (Atomic masses: Pb = $$207.2$$, Cr = $$51.996$$, O = $$16.00$$; Molar mass of PbCrO$$_4$$ = $$323.2 g/$$mol)

$$\frac{207.2}{323.2} \times 100 = 64.11\%$$

Q: How many grams of carbon are present in $$68.1 g of $$sucrose (C$$_{12}H$_{22}O$_{11}$$)? (Molar mass = $$342.30 g/$$mol, mass of C in one mole = $$144.12 g)$$

$$68.1 \times \frac{144.12}{342.30} = 28.7 g$$

Q: A $$342 mg $$sample of benzene is burned and produces $$1156 mg of CO$_2. $$What is the mass percentage of carbon in benzene?

$$\frac{1156 \times 12.01}{342 \times 44.01} \times 100 = 92.3\%$$

Question 1

Which of the following best describes the difference between molecular mass and formula mass?

Accepted Answer

Molecular mass is the sum of atomic masses in a molecule, while formula mass is the sum of atomic masses in a formula unit, which may not be a molecule.

Question 2

Calculate the formula mass of calcium hydroxide, Ca(OH)2, to three significant figures. (Atomic masses: Ca = 40.08, O = 16.00, H = 1.008)

Accepted Answer

$$40.08 + 2 	imes (16.00 + 1.008) = 74.096$$ amu

Question 3

A sample contains $$0.253$$ mol of HNO$$_3. $$What is the mass in grams of this sample? (Molar mass of HNO$$_3$$ = $$63.02 g/$$mol)

Accepted Answer

$$0.253 	imes 63.02 = 15.94 g$$

Question 4

How many moles of CaCO$$_3$$ are present in a $$23.6 g $$sample? (Molar mass of CaCO$$_3$$ = $$100.09 g/$$mol)

Accepted Answer

$$\frac{23.6}{100.09} = 0.236$$ mol

Question 5

The average daily requirement of leucine (C$$_6H$_{14}O$_2N) $$for an adult is $$2.2 g. $$What is this requirement in moles? (Molar mass = $$132.18 g/$$mol)

Accepted Answer

$$\frac{2.2}{132.18} = 0.0166$$ mol

Question 6

The daily requirement of chromium in the human diet is $$1.0 	imes 10$$^{-6}$$ g. How many atoms of chromium does this represent? (Atomic mass of Cr = 51.996$ g/mol, Avogadro's number N_A = 6.022$$ 	imes $$10^{23})$$

Accepted Answer

$$\frac{1.0 	imes 10$$^{-6}$$}{51.996} 	imes 6.022 	imes 10$$^{23}$$ = 1.16 	imes 10$$^{16}$$ atoms

Question 7

What is the percentage composition of lead in PbCrO$$_4$$? (Atomic masses: Pb = $$207.2$$, Cr = $$51.996$$, O = $$16.00$$; Molar mass of PbCrO$$_4$$ = $$323.2 g/$$mol)

Accepted Answer

$$\frac{207.2}{323.2} 	imes 100 = 64.11\%$$

Question 8

How many grams of carbon are present in $$68.1 g of $$sucrose (C$$_{12}H$_{22}O$_{11}$$)? (Molar mass = $$342.30 g/$$mol, mass of C in one mole = $$144.12 g)$$

Accepted Answer

$$68.1 	imes \frac{144.12}{342.30} = 28.7 g$$

Question 9

A $$342 mg $$sample of benzene is burned and produces $$1156 mg of CO$_2. $$What is the mass percentage of carbon in benzene?

Accepted Answer

$$\frac{1156 	imes 12.01}{342 	imes 44.01} 	imes 100 = 92.3\%$$

Question 10

Which of the following is the correct empirical formula for a compound composed of $$34.5\% Na$$, $$21.3\% P$$, and $$42.1\% O$$?

Accepted Answer

Na$$_2P$_2O$_7$$

Question 11

Benzene has the empirical formula CH and a molecular weight of $$78.1$$ amu. What is its molecular formula?

Accepted Answer

C$$_6H$_6$$

Question 12

Which of the following steps is NOT part of determining an empirical formula from percent composition?

Accepted Answer

Using Avogadro's number to convert moles to atoms

Question 13

Propane (C$$_3H$_8) $$burns according to the equation: $$\mathrm{C_3H_8} + 5\mathrm{O_2$} \rightarrow 3\mathrm{CO_2$} + 4\mathrm{H_2O}. $$How many grams of CO$$_2$$ are produced when $$200 g of $$propane is burned? (Molar mass: C$$_3H$_8$$ = $$44.09 g/$$mol, CO$$_2$$ = $$44.01 g/$$mol)

Accepted Answer

$$200 \div 44.09 	imes 3 	imes 44.01 = 599 g$

Question 14

In the reaction $$2\mathrm{NH_3$} + \mathrm{CO_2$} \rightarrow \mathrm{CH_4N_2O} + \mathrm{H_2O}$$, if $$10.0 g NH$_3$$ and $$10.0 g CO$_2$$ are mixed, what is the limiting reactant? (Molar masses: NH$$_3$$ = $$17.03 g/$$mol, CO$$_2$$ = $$44.01 g/$$mol)

Accepted Answer

CO$$_2$$

Question 15

A reaction yields $$13.0 g of $$urea, but the theoretical yield is $$14.0 g. $$What is the percent yield?

Accepted Answer

$$\frac{13.0}{14.0} 	imes 100 = 92.9\%$$

Question 16

Which statement about limiting reactants is correct?

Accepted Answer

The limiting reactant is the one that produces the least amount of product.

Question 17

In the reaction $$\mathrm{$$ZrCl_4$$} + 2\mathrm{Mg} \rightarrow 2\mathrm{$$MgCl_2$$} + \mathrm{Zr}$$, how many moles of Zr can be produced from $$0.20$$ mol ZrCl$$_4$$ and $$0.50$$ mol Mg?

Accepted Answer

0.20 mol

Question 18

Which of the following is the correct formula for calculating percentage yield?

Accepted Answer

$$\frac{	ext{actual yield}}{	ext{theoretical yield}} 	imes 100$$

Calculations with Chemical Formulas and Equations (Chapter 3 Study Notes)

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