# A substance has a molar mass of 270.0 g/mol and a 71.6 g sample of the substance absorbs 4.316 kJ when it melts. Calculate the number of moles in the sample.

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For this question, we won't be using the energy value provided because we can find the number of moles by looking only at the molar mass and the mass of the sample.

Since we are trying to find moles, we want to start with the mass in grams and divide by the molar mass (we will write this as multiplying by the inverse of the molar mass).

71.6 g * (1 mole/270.0 g) = 0.265 moles of the sample.

A common question for this type of problem is also to ask the kJ/mol needed to melt the sample. Now that we know moles of the sample and the energy for that particular sample size, we can also find the kJ/mol by dividing the energy by the number of moles to get

4.316 kJ/0.265 mol = 16.3 kJ/mol

Moles = mass/molar mass

Given

mass = 71.6 g

Molar mass = 270 g/mol

Moles = mass/molar mass

Moles = 71.6/270

Moles = 0.265.