Q: Calculate the percent by mass of sodium in $$\mathrm{Na}_2\mathrm{SO}_4 ($$molar mass = 142.0 g/mol; Na = 23.0 g/mol).

$$\frac{2 \times 23.0}{142.0} \times 100 = 32.4\%$$

Q: How many molecules are in 8.1 \times $$10^{21}$$ molecules of sucrose ($$\mathrm{C}_{12}\mathrm{H}_{22}\mathrm{O}_{11}$$)? What is the mass of this sample? (Molar mass = 342 g/mol, Avogadro's number = $$6.02 \times 10$$^{23}$$)

$$\frac{8.1 \times 10$$^{21}$$}{6.02 \times 10$$^{23}$$} \times 342 = 4.6\ \mathrm{g}$$

Q: A reaction has a theoretical yield of 10.0 g but only 7.5 g of product is actually obtained. What is the percent yield?

$$\frac{7.5}{10.0} \times 100 = 75\%$$

Q: If 1.00 g of glucose ($$\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6) is $$completely combusted, how many grams of water are produced? (Molar mass glucose = 180 g/mol, $$6$$ mol $$\mathrm{H}_2\mathrm{O}$$ per mol glucose, molar mass $$\mathrm{H}_2\mathrm{O} = 18 g/$$mol)

$$1.00 \times \frac{1}{180} \times 6 \times 18 = 0.600\ \mathrm{g}$$

Question 1

Which law is illustrated by the fact that the total mass of reactants equals the total mass of products in a chemical reaction, as shown in the reaction $$2 \mathrm{H}_2(g) + \mathrm{O}_2(g) \rightarrow 2 \mathrm{H}_2\mathrm{O}(l)$$?

Accepted Answer

Law of Conservation of Mass

Question 2

What is the correct balanced equation for the combustion of methane ($$\mathrm{CH}_4) in $$oxygen?

Accepted Answer

$$\mathrm{CH}_4 + 2 \mathrm{O}_2 \rightarrow \mathrm{CO}_2 + 2 \mathrm{H}_2\mathrm{O}$$

Question 3

Why do we balance chemical equations by changing coefficients rather than subscripts?

Accepted Answer

Changing coefficients changes the amount of substances, while changing subscripts changes the identity and properties of the substances.

Question 4

Which of the following is a decomposition reaction?

Accepted Answer

$$2 \mathrm{KClO}_3(s) \rightarrow 2 \mathrm{KCl}(s) + 3 \mathrm{O}_2(g)$$

Question 5

A 0.500 g sample of a compound containing only C, H, and O was combusted, producing 0.7330 g of $$\mathrm{CO}_2$$ and 0.3020 g of $$\mathrm{H}_2\mathrm{O}. $$What is the first step in determining the empirical formula of the compound?

Accepted Answer

Calculate the moles of C and H from the masses of $$\mathrm{CO}_2$$ and $$\mathrm{H}_2\mathrm{O}$$ produced.

Question 6

Which of the following best describes a combination reaction?

Accepted Answer

Two or more reactants combine to form a single product.

Question 7

Calculate the percent by mass of sodium in $$\mathrm{Na}_2\mathrm{SO}_4 ($$molar mass = 142.0 g/mol; Na = 23.0 g/mol).

Accepted Answer

$$\frac{2 	imes 23.0}{142.0} 	imes 100 = 32.4\%$$

Question 8

How many molecules are in 8.1 	imes $$10^{21}$$ molecules of sucrose ($$\mathrm{C}_{12}\mathrm{H}_{22}\mathrm{O}_{11}$$)? What is the mass of this sample? (Molar mass = 342 g/mol, Avogadro's number = $$6.02 	imes 10$$^{23}$$)

Accepted Answer

$$\frac{8.1 	imes 10$$^{21}$$}{6.02 	imes 10$$^{23}$$} 	imes 342 = 4.6\ \mathrm{g}$$

Question 9

Which sample contains the largest mass of chlorine? (a) 0.0100 mol Cl; (b) 0.0100 mol CaCl$$_2$$; (c) 200.0 mg Cl; (d) $$3.01 	imes 10$$^{21}$$ Cl atoms; (e) 3.01$$ 	imes $$10^{21} Cl$_2$$ molecules

Accepted Answer

(b) 0.0100 mol CaCl$$_2$$

Question 10

A reaction has a theoretical yield of 10.0 g but only 7.5 g of product is actually obtained. What is the percent yield?

Accepted Answer

$$\frac{7.5}{10.0} 	imes 100 = 75\%$$

Question 11

If 1.00 g of glucose ($$\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6) is $$completely combusted, how many grams of water are produced? (Molar mass glucose = 180 g/mol, $$6$$ mol $$\mathrm{H}_2\mathrm{O}$$ per mol glucose, molar mass $$\mathrm{H}_2\mathrm{O} = 18 g/$$mol)

Accepted Answer

$$1.00 	imes \frac{1}{180} 	imes 6 	imes 18 = 0.600\ \mathrm{g}$$

Question 12

Which of the following is the limiting reagent when 10 mol $$\mathrm{H}_2$$ reacts with 7 mol $$\mathrm{O}_2 to $$form water according to $$2 \mathrm{H}_2 + \mathrm{O}_2 \rightarrow 2 \mathrm{H}_2\mathrm{O}$$?

Accepted Answer

$$\mathrm{H}_2$$

Question 13

A compound has an empirical formula of CH and a molar mass of 78 g/mol. What is its molecular formula?

Accepted Answer

$$\mathrm{C}_6\mathrm{H}_6$$

Question 14

Which of the following is true about Avogadro’s number?

Accepted Answer

It is the number of particles in one mole of a substance, $$6.02 	imes 10$$^{23}$$.

Question 15

Which of the following is a correct sequence for determining the empirical formula from percent composition?

Accepted Answer

Convert percent to grams, grams to moles, divide by smallest number of moles, write formula.

Question 16

In a reaction, 40.7 g of $$\mathrm{Ca(NO}_3)_2 is $$obtained by reacting 125.0 cm$$^3 of 6.00 $$mol/dm$$^3$$ nitric acid with 38.5 g $$\mathrm{Ca(OH)}_2. $$What is the first step to determine the limiting reagent?

Accepted Answer

Calculate the moles of each reactant using their molar masses and concentrations.

Question 17

Which of the following is NOT a type of chemical reaction discussed in the lesson?

Accepted Answer

Precipitation reaction

Chemical Reaction Equations, Mole Concept, and Stoichiometry: Study Notes for GOB Chemistry

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