Q: Which of the following is the correct formula for calculating percent composition by mass of an element in a compound?

$$\frac{\text{mass of element in 1 mol of compound}}{\text{molar mass of compound}} \times 100$$

Question 1

Which of the following best describes the difference between accuracy and precision in scientific measurements?

Accepted Answer

Accuracy refers to how close measurements are to the true value, while precision refers to how close measurements are to each other.

Question 2

A student measures the mass of a sample three times and records the following values: 2.51 g, 2.50 g, and 2.52 g. The true mass is 2.00 g. Which statement is correct?

Accepted Answer

The measurements are precise but not accurate.

Question 3

Which of the following numbers has exactly three significant figures?

Accepted Answer

0.0300

Question 4

What is the correct answer, with the proper number of significant figures, for the calculation: $$2.34 	imes 1.2$$?

Accepted Answer

$$2.8$$

Question 5

A lake is polluted with $$0.4 \ \mathrm{mg/L} of $$mercury. If the lake has a surface area of $$100 \ \mathrm{m}^2$$ and an average depth of $$20 \ \mathrm{ft}$$, what is the total mass of mercury in kilograms? (1 ft = 0.3048 m)

Accepted Answer

$$0.24 \ \mathrm{kg}$$

Question 6

Which of the following is the correct prefix and symbol for $$10^{-9}$$?

Accepted Answer

nano, n

Question 7

A compound contains 35.0% nitrogen by mass. What is the mass of nitrogen in a $$200 \ \mathrm{g}$$ sample of the compound?

Accepted Answer

$$70.0 \ \mathrm{g}$$

Question 8

Which of the following is the correct empirical formula for a compound with the molecular formula $$C_6H_{12}O_6$?

Accepted Answer

$$CH_2O$$

Question 9

A sample contains $$3.01 	imes 10$$^{23}$$ molecules of CO_2$. How many moles of CO_2$ are present?

Accepted Answer

$$0.50 \ \mathrm{mol}$$

Question 10

Which of the following best explains why the atomic mass of chlorine listed on the periodic table is not a whole number?

Accepted Answer

It is the weighted average of the masses of all naturally occurring isotopes of chlorine.

Question 11

Calculate the percent by mass of nitrogen in ammonium nitrate, $$NH_4NO_3 ($$molar mass = $$80.0 \ \mathrm{g/mol}).$$

Accepted Answer

$$35.0\%$$

Question 12

Which of the following is the correct formula for calculating percent composition by mass of an element in a compound?

Accepted Answer

$$\frac{	ext{mass of element in 1 mol of compound}}{	ext{molar mass of compound}} 	imes 100$$

Question 13

A chemist determines that a compound contains 60.0% C, 4.5% H, and 35.5% O by mass. If the molar mass is approximately $$180 \ \mathrm{g/mol}$$, what is the molecular formula?

Accepted Answer

$$C_6H_{12}O_6$

Question 14

Which of the following is NOT a derived SI unit?

Accepted Answer

meter (m)

Question 15

A student converts $$2.54 \ \mathrm{cm} to $$inches. Which conversion factor should be used?

Accepted Answer

$$\frac{1 \ \mathrm{in}}{2.54 \ \mathrm{cm}}$$

Question 16

Which of the following best describes a systematic error?

Accepted Answer

An error that occurs in the same direction every time due to a flaw in equipment or procedure.

Question 17

If a sample contains $$6.022 	imes 10$$^{23}$$ atoms of aluminum, what is the mass of the sample? (Molar mass of Al = 26.98$$ \ \mathrm{g/mol}$$)

Accepted Answer

$$26.98 \ \mathrm{g}$$

Question 18

Which of the following is the correct empirical formula for benzene, $$C_6H_6$$?

Accepted Answer

$$CH$$

Question 19

A $$1.254 \ \mathrm{g}$$ sample of sulfur reacts with fluorine to form $$5.722 \ \mathrm{g} of$$ $$SF_6. $$What is the value of $$x in$$ $$SF_x$$?

Accepted Answer

$$6$$

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