Empirical Formulas: Quiz

Empirical Formulas: Quiz

7 questions

1. Question 1EasyWhat is the empirical formula of glucose, $\ce{C6H12O6}$?

   A$\ce{C6H12O6}$B$\ce{C2H4O2}$C$\ce{CH2O}$D$\ce{CHO}$
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   Answer: $\ce{CH2O}$

   Divide every subscript by the largest common factor, 6: $\ce{C6H12O6}$ → $\ce{CH2O}$. $\ce{C2H4O2}$ has the right ratio but is not the simplest one.

2. Question 2EasyWhich of these formulas is already an empirical formula?

   A$\ce{C2H6}$B$\ce{N2O4}$C$\ce{CO2}$D$\ce{C4H10}$
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   Answer: $\ce{CO2}$

   In $\ce{CO2}$ the ratio 1 : 2 cannot be simplified. The others can: $\ce{C2H6}$ → $\ce{CH3}$, $\ce{N2O4}$ → $\ce{NO2}$, $\ce{C4H10}$ → $\ce{C2H5}$.

3. Question 3MediumAfter dividing by the smallest number of moles, you get Fe : O = 1 : 1.50. What is the empirical formula?

   A$\ce{FeO}$B$\ce{FeO2}$C$\ce{Fe2O3}$D$\ce{Fe3O2}$
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   Answer: $\ce{Fe2O3}$

   A ratio ending in .5 must be multiplied by 2: 1 : 1.50 becomes 2 : 3, so $\ce{Fe2O3}$. Rounding 1.5 up to 2 would give the wrong formula, $\ce{FeO2}$.

4. Question 4MediumA compound is 74.87% C and 25.13% H by mass. What is its empirical formula?

   A$\ce{CH}$B$\ce{CH3}$C$\ce{CH4}$D$\ce{C3H}$
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   Answer: $\ce{CH4}$

   Moles in 100 g: C = 74.87 ÷ 12.01 = 6.234; H = 25.13 ÷ 1.008 = 24.93. Divide by 6.234: C = 1.00, H = 4.00, so $\ce{CH4}$. The answer $\ce{C3H}$ comes from using the percentages directly instead of moles.

5. Question 5MediumA compound has empirical formula $\ce{CH2}$ and molar mass 42.08 g/mol. What is its molecular formula?

   A$\ce{CH2}$B$\ce{C2H4}$C$\ce{C3H6}$D$\ce{C6H12}$
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   Answer: $\ce{C3H6}$

   Empirical formula mass = 12.01 + 2(1.008) = 14.03 g/mol. $n = 42.08 \div 14.03 = 3.00$, so the molecular formula is $\ce{C3H6}$.

6. Question 6MediumBenzene has empirical formula $\ce{CH}$ and molar mass 78.11 g/mol. What is its molecular formula?

   A$\ce{C2H2}$B$\ce{C4H4}$C$\ce{C6H6}$D$\ce{C8H8}$
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   Answer: $\ce{C6H6}$

   Empirical formula mass = 12.01 + 1.008 = 13.02 g/mol. $n = 78.11 \div 13.02 = 6.00$, so $\ce{C6H6}$.

7. Question 7EasyWhy do you divide every mole value by the smallest one?

   ATo convert grams into molesBTo make the smallest value 1 and find the ratio between elementsCTo find the molar mass of the compoundDTo turn the ratio into percentages
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   Answer: To make the smallest value 1 and find the ratio between elements

   Dividing by the smallest value scales the numbers so the smallest becomes exactly 1. The others then show how many atoms of each element there are for every one atom of that element.