AP Chemistry 1.3: Elemental Composition

Practice AP Chemistry 1.3 questions on converting elemental masses to mole ratios and determining empirical formulas for pure substances.

[Maximum number: 2]

Answer the following questions about ascorbic acid (vitamin C).

A student combusts a sample of ascorbic acid, $\mathrm{C}_{x} \mathrm{H}_{y} \mathrm{O}_{x}$ , to determine its chemical

composition. The only products of the reaction are 0.2400 mol of $\mathrm{CO}_{2}$ and 2.883 g of $\mathrm{H}_{2} \mathrm{O}$ .

The mole ratio of carbon (C) to oxygen (O) is 1:1 in ascorbic acid. Based on this

information and your answer to part A (i), determine the empirical formula of ascorbic

acid.

(ii) For the correct calculated number of moles of H (may be implicit):

$0.1600 \mathrm{~mol} \mathrm{H}_{2} \mathrm{O} \times \frac{2 \mathrm{~mol} \mathrm{H}^{\mathrm{H}}}{1 \mathrm{~mol} \mathrm{H}_{2} \mathrm{O}}=0.3200 \mathrm{~mol} \mathrm{H}$

For the correct empirical formula.

Examples of acceptable responses may include the following:

- x: y: z= (moles of C):(moles of H):(moles of O)

x: y: z=0.2400: 0.3200: 0.2400=3: 4: 3

Therefore, the empirical formula of ascorbic acid is $\mathrm{C}_{3} \mathrm{H}_{4} \mathrm{O}_{3}$ .

- $0.2400 \mathrm{~mol} \mathrm{CO}_{2} \times \frac{1 \mathrm{~mol} \mathrm{C}}{1 \mathrm{~mol} \mathrm{CO}_{2}}=0.2400 \mathrm{~mol} \mathrm{C}$ $\frac{0.3200 \mathrm{~mol} \mathrm{H}}{0.2400 \mathrm{~mol} \mathrm{C}}=\frac{4 \mathrm{H}}{3 \mathrm{C}}$

Given that the ratio of C:O is 1:1, the empirical formula of ascorbic acid is

$\mathrm{C}_{3} \mathrm{H}_{4} \mathrm{O}_{3}.$

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AP Chemistry 1.3: Elemental Composition

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1.3 Elemental Composition of Pure Substances question 1