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How do you calculate the number of moles from density?

Assume you are given this question: If the of acetic anhydride, C₄H₆O₃, is 1.082 g/mL, how many moles are in 10.0 mL of the sample?

Somehow, you have to get from millilitres to moles. 1. You know the density, so you can convert millilitres to grams. 2. How do you convert grams to moles? You use the molar mass.

Let’s see how this works. 1. Convert millilitres to grams. 10.0 mL × (1.082 g/1 mL) = 10.8 g 2. Convert grams to moles.

You have to work out the molar mass of C₄H₆O₃. From , you calculate the molar mass to be 102.1 g/mol. Then, 10.8 g × (1 mol/102.1 g) = 0.106 mol

Notice how, in each step, we set up the conversion factor so that the units cancel to give the correct units for the answer. You should always include the units in your calculations. That way, if the units do not work out, you know that you have made a mistake.

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