Sarah measures out 151 grams of SO2. How many moles is this? Express your answer to three significant figures.

Explanation:

We are asked to convert grams to moles. We will use the molar mass and dimensional analysis to perform this conversion.

The molar mass is the mass of 1 mole of a substance. These values are found on the Periodic Table because they are equivalent to the atomic masses, but the units are grams per mole instead.

We are given a mass of sulfur dioxide (SO₂). Look up the molar masses of the individual elements.

Notice that the formula of the compound contains a subscript. The subscript after O means there are 2 moles of oxygen in 1 mole of sulfur dioxide. We must multiply oxygen's molar mass before adding sulfur's.

Now we will use dimensional analysis to convert grams to moles. From the molar mass, we know there are 64.068 grams of sulfur dioxide per mole, so we can set up a ratio.

We are converting 151 grams to moles, so we multiply by this value.

Flip the ratio so the units of grams of sulfur dioxide cancel.

The original measurement of grams (151) has 3 significant figures, so our answer must have the same. For the number we calculated, that is the hundredth place. The 6 in the thousandth place tells us to round the 5 in the hundredth up to a 6.

151 grams of sulfur dioxide is approximately 2.36 moles of sulfur dioxide.

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