Question 1

Which of the following best describes an empirical formula?

Accepted Answer

The formula that shows the simplest whole number ratio of elements in a compound.

Question 2

Given the molecular formula $$C_6H_{12}O_6$, what is its empirical formula?

Accepted Answer

$$CH_2O$$

Question 3

A compound contains 29.08% Na, 40.58% S, and 30.34% O by mass. What is the empirical formula of the compound?

Accepted Answer

$$Na_2S_2O_3$$

Question 4

If you have 1 mol of $$H_2O$$, how many moles of hydrogen atoms are present?

Accepted Answer

2 mol

Question 5

Which instrument is commonly used to determine the percent composition of elements in a compound?

Accepted Answer

Mass spectrometer

Question 6

A sample contains 40.58 g of sulfur. How many moles of sulfur are present? (Atomic mass of S = 32.07 g/mol)

Accepted Answer

$$\frac{40.58}{32.07} = 1.266$$ mol

Question 7

What is the correct sequence of steps to determine the empirical formula from percent composition data?

Accepted Answer

Convert percent to grams, convert grams to moles, divide by smallest number of moles, round to nearest whole number.

Question 8

A compound is found to contain 30.34 g of oxygen. How many moles of oxygen are present? (Atomic mass of O = 16.00 g/mol)

Accepted Answer

$$\frac{30.34}{16.00} = 1.896$$ mol

Question 9

If the mole ratio of Na:S:O in a compound is 2:2:3, what is the empirical formula?

Accepted Answer

$$Na_2S_2O_3$$

Question 10

Which of the following statements about molecular and empirical formulas is correct?

Accepted Answer

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

Question 11

A mass spectrometer separates ions based on which property?

Accepted Answer

Their mass-to-charge ratio ($$\frac{m}{z})$$

Question 12

If a compound has a percent composition of 20% Na, 30% S, and 50% O, what is the first step in determining its empirical formula?

Accepted Answer

Convert percent to grams (assume 100 g sample)

Question 13

Which of the following is NOT a correct step in finding the empirical formula from percent composition?

Accepted Answer

Add the atomic masses of all elements together before dividing by the smallest number of moles.

Question 14

A compound contains 29.08 g Na, 40.58 g S, and 30.34 g O. What is the mole ratio of Na:S:O?

Accepted Answer

1.266 : 1.266 : 1.896

Question 15

If the calculated mole ratio for an element is 1.57, what should you do to obtain a whole number ratio?

Accepted Answer

Multiply all ratios by 2

Question 16

Which formula represents how a compound appears in nature?

Accepted Answer

Molecular formula

Question 17

A student analyzes a compound and finds the following mole ratios: 1.25 : 2.50 : 3.75. What is the empirical formula after converting to whole numbers?

Accepted Answer

$$A_5B_{10}C_{15}$$

Question 18

Which of the following is a correct application of percent composition in a real-world scenario?

Accepted Answer

Determining the purity of a pharmaceutical compound.

Question 19

If a mass spectrometer shows three peaks for a sample, what does this indicate?

Accepted Answer

The sample contains three different elements or isotopes.

Question 20

Which of the following is the correct formula for sodium thiosulfate based on the empirical formula calculation shown?

Accepted Answer

$$Na_2S_2O_3$$

Percent Composition and Empirical Formula in General Chemistry

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