Question

An Alka-Seltzer tablet contains 324 mg of aspirin (C9H8O4), 1904 mg of NaHCO3, and 1000. mg of citric acid (H3C6H5O7). (The last two compounds react with each other to provide the "fizz," bubbles of CO2, when the tablet is put into water.) Calculate the amount (moles) of each substance in the tablet.

Answer 1

Number of moles of a substance can be calculated from mass of the substance and its molar mass as follows:

$n=\frac{m}{M}$

Here, n is number of moles, m is mass and M is molar mass.

For aspirin: 324 mg

Molecular formula is $C_{9}H_{8}O_{4}$ and molar mass of aspirin is 180.157 g/mol, thus, number of moles will be:

$n=\frac{324\times 10^{-3} g}{180.157 g/mol}=0.0018 mol$

For sodium hydrogen carbonate: 1904 mg

Molecular formula is $NaHCO_{3}$ and molar mass is 84.007 g/mol, thus, number of moles will be:

$n=\frac{1904\times 10^{-3} g}{84.007 g/mol}=0.0226 mol$

For citric acid: 1000 mg

Molecular formula is $H_{3}C_{6}H_{5}O_{7}$ and molar mass is 192.124 g/mol, thus, number of moles will be:

$n=\frac{1000\times 10^{-3} g}{192.124 g/mol}=0.0052 mol$

Therefore, number of moles of aspirin, $NaHCO_{3}$ and citric acid in the tablet is 0.0018 mol, 0.0226 mol and 0.0052 mol respectively.