SAT Subject Test in Chemistry - Molar Mass

First, calculate the molar mass of heptane, which is

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

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:

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

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

Molar Mass

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