Q: Chlorine has two naturally occurring isotopes: chlorine-35 (75.77%) and chlorine-37 (24.23%). What is the atomic mass of chlorine?

$$(0.7577 \times 35.00) + (0.2423 \times 37.00) = 35.49$$

Q: A sample contains 0.551 g of potassium (K). How many atoms of potassium are present? (Atomic mass of K = 39.10 g/mol, $$N_A = 6.022 \times 10$$^{23}$$)

$$\frac{0.551}{39.10} \times 6.022 \times 10$$^{23}$$ = 8.48 \times 10$$^{21}$$ atoms

Q: What is the percent composition by mass of oxygen in sulfuric acid ($$H_2SO_4$$)? (Molar mass = 98 g/mol)

$$\frac{64}{98} \times 100 = 65.3\%$$

Q: A laboratory experiment yields 11.2 g of $$POCl_3$$ from 11.0 g of $$PCl_3$$ and 1.3 g of $$O_2. $$What is the percent yield if the theoretical yield is 12.0 g?

$$\frac{11.2}{12.0} \times 100 = 93.3\%$$

Question 1

Which statement best describes the atomic mass unit (amu)?

Accepted Answer

It is defined as exactly one-twelfth the mass of a carbon-12 atom.

Question 2

Chlorine has two naturally occurring isotopes: chlorine-35 (75.77%) and chlorine-37 (24.23%). What is the atomic mass of chlorine?

Accepted Answer

$$(0.7577 	imes 35.00) + (0.2423 	imes 37.00) = 35.49$$

Question 3

Avogadro's number ($$N_A) is $$defined as:

Accepted Answer

The number of molecules in 1 mole of a substance.

Question 4

A sample contains 0.551 g of potassium (K). How many atoms of potassium are present? (Atomic mass of K = 39.10 g/mol, $$N_A = 6.022 	imes 10$$^{23}$$)

Accepted Answer

$$\frac{0.551}{39.10} 	imes 6.022 	imes 10$$^{23}$$ = 8.48 	imes 10$$^{21}$$ atoms

Question 5

What is the percent composition by mass of oxygen in sulfuric acid ($$H_2SO_4$$)? (Molar mass = 98 g/mol)

Accepted Answer

$$\frac{64}{98} 	imes 100 = 65.3\%$$

Question 6

A compound contains 24.75% K, 34.77% Mn, and 40.51% O by mass. What is its empirical formula?

Accepted Answer

$$KMnO_4$$

Question 7

Which of the following is the correct balanced equation for the reaction of iron(II) oxide with carbon monoxide to yield iron and carbon dioxide?

Accepted Answer

$$Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2$$

Question 8

How many moles of $$NO_2$$ are formed when 1 mole of $$O_2$$ reacts completely with $$NO$$ according to $$2NO + O_2$ \rightarrow 2NO_2$$?

Accepted Answer

2 moles

Question 9

If 87.3 g of magnesium (Mg) are present, how many moles of magnesium is this? (Atomic mass of Mg = 24.31 g/mol)

Accepted Answer

$$\frac{87.3}{24.31} = 3.59$$ moles

Question 10

Methanol ($$CH_3OH) $$burns in air according to $$2CH_3OH + 3O_2 \rightarrow 2CO_2 + 4H_2O. If 32.0 g of $$methanol are used up, how many grams of water are produced? (Molar mass of $$CH_3OH = 32.04 g/$$mol, $$H_2O = 18.02 g/$$mol)

Accepted Answer

36.04 g

Question 11

In a reaction, 124 g of Al are reacted with 601 g of $$Fe_2O_3. $$What is the limiting reagent in the formation of $$Al_2O_3$$?

Accepted Answer

Aluminum (Al)

Question 12

A laboratory experiment yields 11.2 g of $$POCl_3$$ from 11.0 g of $$PCl_3$$ and 1.3 g of $$O_2. $$What is the percent yield if the theoretical yield is 12.0 g?

Accepted Answer

$$\frac{11.2}{12.0} 	imes 100 = 93.3\%$$

Question 13

Which of the following best describes a limiting reagent in a chemical reaction?

Accepted Answer

The reactant that is used up first and limits the amount of product formed.

Question 14

Calculate the number of moles of chloroform ($$CHCl_3) in 198 g of $$chloroform. (Molar mass = 119.37 g/mol)

Accepted Answer

$$\frac{198}{119.37} = 1.66$$ moles

Question 15

A sphere of iron has a radius of 3.4 cm. If the density of iron is 7.86 g/cm$$^3$$, how many iron atoms are in the sphere? (Atomic mass of Fe = 55.85 g/mol, $$N_A = 6.022 	imes 10$$^{23}$$)

Accepted Answer

$$\frac{4}{3} \pi (3.4)^3$ 	imes 7.86 = 1,303 g$$; $$\frac{1,303}{55.85} 	imes 6.022 	imes 10$$^{23}$$ = 1.40 	imes 10$$^{25}$$ atoms

Question 16

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

Accepted Answer

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

Question 17

If 99.8 g of $$O_2$$ reacts with excess $$H_2 to $$produce $$H_2O$$ according to $$2H_2 + O_2$ \rightarrow 2H_2O$$, what mass of water is produced? (Molar mass of $$O_2 = 32.00 g/$$mol, $$H_2O = 18.02 g/$$mol)

Accepted Answer

$$\frac{99.8}{32.00} = 3.12$$ mol $$O_2$$; $$3.12 	imes 2 = 6.24$$ mol $$H_2O$$; $$6.24 	imes 18.02 = 112.5 g$$

Compounds, Mole, and Stoichiometry: Fundamental Concepts and Calculations

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