In this problem we want to find the mass percent of chromium in our

aluminum dichromate.

So here we're going to first find the molar mass of the aluminum dichromate.

We see that we have two aluminum atoms, each with a mass of 26.98.

So that gives us 53.96 from the aluminum.

We have six chromium atoms, each with a molar mass of 52.00,

which gives us a mass of 312.

And we have 21 oxygen atoms.

Remember that I have to take the 3 times 7 because I have 21 oxygens,

each with a mass of 16, and that gives us a total of 336 for the oxygen.

When I add all these values together, I end up with

701.96 grams per mole for the molar mass of aluminum dichromate.

And then I want to find the percent of chromium.

Remember that percent is always equal to part over the whole times 100.

And so here I'm going to get my percent chromium, by looking

at the mass from the chromium which is 312 divided by 701.96 times 100.

And when I do that I find that the percent chromium equals 44.45%.

If I did the same process for the aluminum and the oxygen,

what I would find is that the 3 percentages sum, should sum to 100% total.