SAT II Chemistry : Molar Mass

Study concepts, example questions & explanations for SAT II Chemistry

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Example Questions

Example Question #1 : Molar Mass

What is the approximate percentage of carbon by mass in heptane, which has the molecular formula \(\displaystyle C_7H_{16}\)?

Possible Answers:

\(\displaystyle 6\%\)

\(\displaystyle 16\%\)

\(\displaystyle 36\%\)

\(\displaystyle 72\%\)

\(\displaystyle 84\%\)

Correct answer:

\(\displaystyle 84\%\)

Explanation:

First, calculate the molar mass of heptane, which is

\(\displaystyle 7(12\text{ amu}) + 16(1\text{ amu}) = 100\text{ amu or } \frac{\text{g}}{\text{mol}}\)

We can calculate the percentage of carbon by mass in heptane by creating a fraction of the mass of carbon over the mass of the entire molecule and converting it into a percentage:

\(\displaystyle \frac{\text{Mass of C in C}_7\text{H}_{16}}{\text{Mass of C}_7\text{H}_{16}}=\frac{7(12\text{ amu})}{100\text{ amu}}=\frac{84}{100}\times100\%=84\%\text{ C}\)

For every 100 g of heptane, 84 g must come from carbon. This means that the percentage of carbon by mass is \(\displaystyle 84\%.\)

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