Q: A 12.0 g sample of CoCl$$_2$$ contains how many formula units? (Molar mass of CoCl$$_2 = 129.84 g/$$mol, Avogadro's number = $$6.022 \times 10$$^{23}$$ mol$$^{-1}$$)

$$5.57 \times 10$$^{22}$$ formula units

Q: Which equation is used to determine the molecular formula from the empirical formula and molar mass?

$$n = \frac{\text{molar mass}}{\text{empirical\ formula\ molar\ mass}}$$

Question 1

A 12.0 g sample of CoCl$$_2$$ contains how many formula units? (Molar mass of CoCl$$_2 = 129.84 g/$$mol, Avogadro's number = $$6.022 	imes 10$$^{23}$$ mol$$^{-1}$$)

Accepted Answer

$$5.57 	imes 10$$^{22}$$ formula units

Question 2

What is the mass percent of oxygen in CO$$_2$$? (Atomic masses: C = 12.01 g/mol, O = 16.00 g/mol)

Accepted Answer

72.71%

Question 3

A 7.00 g sample of CCl$$_3F $$contains how many grams of chlorine? (Atomic mass Cl = 35.45 g/mol, Molar mass CCl$$_3F = 137.37 g/$$mol)

Accepted Answer

4.87 g

Question 4

Which of the following best describes an empirical formula?

Accepted Answer

The simplest whole-number ratio of atoms of each element in a compound

Question 5

A compound contains 88.16% C and 11.84% H by mass. What is its empirical formula? (Atomic masses: C = 12.01 g/mol, H = 1.008 g/mol)

Accepted Answer

C$$_5H$_8$$

Question 6

A 0.358 g sample of chromium reacts with oxygen to form 0.523 g of chromium oxide. What is the empirical formula of the oxide? (Atomic masses: Cr = 52.00 g/mol, O = 16.00 g/mol)

Accepted Answer

Cr$$_2O$_3$$

Question 7

A compound has an empirical formula of CH$$_2$$ and a molar mass of 84 g/mol. What is its molecular formula? (Empirical formula mass = 14.03 g/mol)

Accepted Answer

C$$_6H$_{12}$$

Question 8

A 1.00 g sample of an organic compound produces 2.44 g CO$$_2$$ and 0.999 g H$$_2O $$upon complete combustion. What is the empirical formula of the compound? (Atomic masses: C = 12.01 g/mol, H = 1.008 g/mol, O = 16.00 g/mol)

Accepted Answer

C$$_4H$_8O$$

Question 9

Combustion analysis of 2.56 g naphthalene produces 8.80 g CO$$_2$$ and 1.44 g H$$_2O. $$Does naphthalene contain any oxygen?

Accepted Answer

No, it contains only carbon and hydrogen.

Question 10

A 4.93 g sample of Epsom salts (MgSO$$_4$$·xH$$_2O) is $$heated to drive off water, leaving 2.41 g of anhydrous MgSO$$_4. $$What is the value of x? (Molar mass MgSO$$_4 = 120.37 g/$$mol, H$$_2O = 18.016 g/$$mol)

Accepted Answer

7

Question 11

Which equation is used to determine the molecular formula from the empirical formula and molar mass?

Accepted Answer

$$n = \frac{	ext{molar mass}}{	ext{empirical\ formula\ molar\ mass}}$$

Question 12

A compound contains only C, H, and N. Combustion of 204.2 mg of the compound produces 439.5 mg CO$$_2$$, 251.1 mg H$$_2O$$, and an unknown mass of nitrogen compounds. What is the empirical formula?

Accepted Answer

C$$_5H$_{14}N$_2$$

Question 13

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

Accepted Answer

Divide the mass of each element by the total mass of the compound.

Question 14

A hydrocarbon sample weighing 12.501 g produces 38.196 g CO$$_2$$ and 18.752 g H$$_2O $$upon combustion. What is the empirical formula of the hydrocarbon?

Accepted Answer

C$$_5H$_{12}$$

Question 15

Which of the following best describes the process of combustion analysis as shown in the diagram?

Accepted Answer

An unknown compound is burned in oxygen, and the resulting H$$_2O $$and CO$$_2$$ are absorbed and weighed.

Question 16

If a compound has a percent composition of 61.31% C, 5.14% H, 10.21% N, and 23.33% O, what is its empirical formula?

Accepted Answer

C$$_7H$_7NO$_2$$

Question 17

Which of the following is true about the relationship between empirical and molecular formulas?

Accepted Answer

The molecular formula is always a whole-number multiple of the empirical formula.

chapter 4, gen chem, n) Chemical Formulas: Empirical, Molecular, and Combustion Analysis

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