# How many oxygen atoms are present in 3.99g of sulfur dioxide?

txmedteach | High School Teacher | (Level 3) Associate Educator

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To answer this, we'll need to figure out how many molecules of sulfur dioxide are present, then we can figure out how many oxygen atoms there are.

Step 1: Find the number of sulfur dioxide molecules

The best way to do this is to recognize two important relationships

1) For every mole of sulfur dioxide, we have the molar mass of sulfure dioxide in grams.

2) For every mole of sulfure dioxide, we have 6.02*10^23 molecules of sulfure dioxide.

To find the number of moles of SO2 (sulfure dioxide from here on out to save space), we need to first figure out the molar mass. We'll look at the periodic table and add the molar mass of sulfur (32.1 g/mol, see first link) with twice the molar mass of oxygen (16.0 g/mol, see second link) (because we have 2 oxygens in the molecular formula).

Molar Mass (SO2) = Molar Mass (sulfur) + 2*Molar Mass (oxygen)

Molar Mass (SO2) = 32.1 + (2*16.0) = 64.1 g/mol

(Side Note) Just in case you were wondering, we round off at the 3rd significant figure because we were only given 3 significant figures in the problem.

Now that we know our molar mass, we can determine the number of moles in 3.99 g of SO2:

moles SO2 = mass on hand (SO2) / molar mass (SO2)

moles SO2 = 3.99 / 64.1 = 0.0622 mol

(Side Note) Notice again, 3 significant figures!

Now, we can proceed to relationship (2) that we talked about above: linking the number of moles of SO2 to the number of molecules of SO2. We do this by multiplying the number of moles we have by Avagadro's number (per the definition of the unit "mole," see the third link below):

molecules SO2 = moles SO2 * Avagadro's number

molecules SO2 = 0.0622 * (6.02 * 10^23)

molecules SO2 = 3.74 * 10^22 molecules

That is a certainly a lot of molecules (to three significant figures), and it's how many SO2 molecules we have in that sample!

Now, we can move on to determining the number of oxygen atoms.

Step 2: Find the number of oxygen atoms

Now, we're going to take apart that SO2 molecule. We know it contains 1 sulfur atom and 2 oxygen atoms. Therefore, the number of oxygen atoms will be double the number of molecules.

Put in equation form:

Atoms (oxygen) = Molecules (SO2) * 2

We have already found the number of SO2 molecules, so let's finish this out:

Atoms (oxygen) = Molecules (SO2) * 2

Atoms (oxygen) = 3.74 * 10^22 * 2 = 7.48 * 10^22 atoms

We finally  have our answer (again to 3 significant figures):

7.48 * 10^22 atoms of oxygen

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