Question 1

A compound is found to contain 48.64% carbon and 51.36% oxygen by mass. What is the empirical formula of this compound?

Accepted Answer

$$\mathrm{CO}$$

Question 2

Elemental analysis of a sample of an ionic compound showed 2.82 g of Na, 4.35 g of Cl, and 7.83 g of O. What is the empirical formula of the compound?

Accepted Answer

$$\mathrm{NaClO}_3$$

Question 3

A compound composed of carbon, hydrogen, and chlorine contains $$4.19 	imes 10$$^{23}$$ hydrogen atoms. If 9.00 g of the compound also contains 55.05% chlorine by mass, what is the empirical formula?

Accepted Answer

$$\mathrm{CH}_2\mathrm{Cl}$$

Question 4

Given the empirical formula $$\mathrm{NPC}_2$$ and a molar mass of 347.64 g/mol, what is the molecular formula of the compound?

Accepted Answer

$$\mathrm{N}_4\mathrm{P}_4\mathrm{C}_8$$

Question 5

Cortisol (M = 362.47 g/mol), a known steroid hormone, is found to contain 69.6% carbon, 8.34% hydrogen, and 22.1% oxygen by mass. What is its molecular formula?

Accepted Answer

$$\mathrm{C}_{21}\mathrm{H}_{30}\mathrm{O}_5$$

Question 6

If a sample of sodium chloride, NaCl, contains 73.1 kg, what is its number of formula units?

Accepted Answer

$$7.54 	imes 10$$^{26}$$

Question 7

Calculate the number of oxygen atoms found in 783.9 g of $$\mathrm{NiCl}_2 \cdot 6\mathrm{H}_2\mathrm{O}.$$

Accepted Answer

$$1.58 	imes 10$$^{25}$$

Question 8

If the density of water is 1.00 g/mL at 25°C, calculate the number of water molecules found in $$1.50 	imes 10^3$ $$\mu$$L of water.

Accepted Answer

$$5.02 	imes 10$$^{22}$$

Question 9

A cylindrical copper wire is used for fences around a house. The copper wire has a diameter of 0.0750 in. How many copper atoms are found in a 5.160 cm piece? (Density of copper = 8.96 g/cm$$^3$$; $$V = \pi r^2$ h)$$

Accepted Answer

$$6.89 	imes 10$$^{22}$$

Question 10

The density of the sun is 1.41 g/cm$$^3$$ and its volume is $$1.41 	imes 10$$^{27}$$ m^3$. How many hydrogen molecules are in the sun if we assume all the mass is hydrogen gas?

Accepted Answer

$$2.98 	imes 10$$^{56}$$

Question 11

Calculate the mass percent of carbon within sodium carbonate, $$\mathrm{Na}_2\mathrm{CO}_3.$$

Accepted Answer

$$11.3\%$$

Question 12

Determine the percent composition of nitrogen and oxygen in nitrogen dioxide, $$\mathrm{NO}_2.$$

Accepted Answer

30.4% N, 69.6% O

Question 13

How many grams of chlorine gas are there in 2.34 moles?

Accepted Answer

$$165.19 g$$

Question 14

How many grams of $$\mathrm{Cl}_2$$ contain $$9.25 	imes 10$$^{24}$$ molecules of $$\mathrm{Cl}_2$$?

Accepted Answer

$$1,090 g$$

Question 15

Which of the following compounds would NOT be associated with the term 'formula unit'?

Accepted Answer

$$\mathrm{BF}_3$$

Question 16

How many moles of chlorine gas are there in $$8.33 	imes 10$$^{23}$$ molecules?

Accepted Answer

$$1.38 	imes 10$$^{-1}$$ mol

Question 17

A pure compound is found to contain 72.2% C, 8.50% H, and the remainder as O. If 0.250 moles of the compound weighs 41.55 g, what is the molecular formula?

Accepted Answer

$$\mathrm{C}_{10}\mathrm{H}_{12}\mathrm{O}_2$$

Empirical and Molecular Formulas, The Mole, and Mass Percent in Chemistry

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