Question

How many molecules are in 31.9 g of hexane?

Answer 1

I will assume you mean `C_6H_14`

Multiply Avagadro's Number times the amount of hexane and divide by the molar mass of hexane.

(6.022 X `10^23`) * 31.9 g / (molar mass of hexane)

Explanation:

Note: If you were talking about Cyclohexane, the formula would be `C_6H_12`

The formula for obtaining the number of particles given the amount of sample in grams follows this formula:

(6.022 X `10^23`) X (grams of sample) / (molar mass of hexane)

In performing a unit conversion, you are actually doing the same thing to the units that you are doing to the values.

(particles / mole) * (g) / (g/mole)

Source: Generated by David M. Using Microsoft PowerPoint

The moles and the grams cancel, leaving an answer in particles.

You may ask why I say "particles" and not "molecules"? The reason is so the SAME calculation may be performed for ionic compounds, ions, or other types of chemical identities.