Q: Which equation correctly calculates the percent composition of an element in a compound?

$$\frac{(\text{number of atoms})(\text{atomic weight})}{\text{formula weight}} \times 100$$

Question 1

Which principle is the foundation of stoichiometry and states that the total mass remains constant during a chemical reaction?

Accepted Answer

Law of Conservation of Mass

Question 2

In the balanced equation $$\mathrm{CH_4$(g) + 2O_2(g) \rightarrow CO_2$(g) + 2H_2O(g)}$$, what do the coefficients represent?

Accepted Answer

The number of molecules or moles involved in the reaction

Question 3

Why is it incorrect to change subscripts instead of coefficients when balancing chemical equations?

Accepted Answer

Changing subscripts alters the identity and properties of the compound

Question 4

Which type of reaction is represented by $$\mathrm{2Mg(s) + O_2$(g) \rightarrow 2MgO(s)}$$?

Accepted Answer

Combination reaction

Question 5

A compound contains 61.31% C, 5.14% H, 10.21% N, and 23.33% O by mass. What is the empirical formula?

Accepted Answer

$$\mathrm{C_7H_7NO_2}$$

Question 6

What is the formula weight of $$\mathrm{$$CaCl_2$$}$$?

Accepted Answer

$$110.99\ \mathrm{amu}$$

Question 7

How many atoms are in one millionth of a mole of a noble gas?

Accepted Answer

$$6.02 	imes 10$$^{17}$$

Question 8

Which equation correctly calculates the percent composition of an element in a compound?

Accepted Answer

$$\frac{(	ext{number of atoms})(	ext{atomic weight})}{	ext{formula weight}} 	imes 100$$

Question 9

If the empirical formula of a compound is $$\mathrm{CH}$$ and its molar mass is $$78\ \mathrm{g/mol}$$, what is its molecular formula?

Accepted Answer

$$\mathrm{C_6H_6}$$

Question 10

Which of the following is a decomposition reaction?

Accepted Answer

$$\mathrm{$$CaCO_3$$(s) \rightarrow CaO(s) + CO_2$(g)}$$

Question 11

What is the percent yield if the theoretical yield of adipic acid is $$40.0\ \mathrm{g}$$ and the actual yield is $$33.5\ \mathrm{g}$$?

Accepted Answer

$$83.75\%$$

Question 12

In a combustion analysis, how is the amount of oxygen in a compound determined?

Accepted Answer

By subtracting the masses of C and H from the total mass

Question 13

Which reactant is the limiting reactant when $$10.0\ \mathrm{g} of$$ $$\mathrm{C_6H_6}$$ and $$30.0\ \mathrm{g} of$$ $$\mathrm{Br_2$}$$ react according to $$\mathrm{C_6H_6 + 2Br_2 \rightarrow C_6H_4Br_2 + 2HBr}$$?

Accepted Answer

$$\mathrm{Br_2$}$$

Question 14

What is the molar mass of $$\mathrm{Na_3PO_4}$$?

Accepted Answer

$$164\ \mathrm{g/mol}$$

Question 15

Which of the following best describes a combustion reaction?

Accepted Answer

A rapid reaction with oxygen producing a flame

Question 16

If $$1.00\ \mathrm{g} of $$glucose ($$\mathrm{C_6H_{12}O_6$}) is $$completely reacted, how many grams of water ($$\mathrm{H_2O}) $$can be produced? (Molar mass of glucose = $$180\ \mathrm{g/mol}$$, molar mass of water = $$18\ \mathrm{g/mol})$$

Accepted Answer

$$0.60\ \mathrm{g}$$

Question 17

Which step is NOT part of determining an empirical formula from percent composition?

Accepted Answer

Using molar mass to find grams of each element

Question 18

What is the theoretical yield of $$\mathrm{C_6H_{10}O_4$} ($$adipic acid) if $$25.0\ \mathrm{g} of $$cyclohexane ($$\mathrm{C_6H_{12}}) is $$used and is the limiting reactant? (Molar mass of cyclohexane = $$84\ \mathrm{g/mol}$$, molar mass of adipic acid = $$146\ \mathrm{g/mol})$$

Accepted Answer

$$43.5\ \mathrm{g}$$

Question 19

Which statement about ionic compounds is correct?

Accepted Answer

They use empirical formulas and formula weights, not molecular weights

Stoichiometry: Calculations with Chemical Formulas and Equations

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