# Find molar composition from percent by mass

50.0 g of an oxide of Chromium (Cr) was found to contain 13.28g of potassium (K), 17.68 g of Chromium (Cr) and 19.04 g of oxygen (O). Find the simplest formula...

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## SOLUTION This solution is **FREE** courtesy of BrainMass!

A chemist cannot relate masses of compounds directly, he/she must always relate molar values.

One must first convert these mass values into mole values by dividing each value by their respective molar masses.

13.28 g K

--------- = 0.3400 mol

39.10 g/mol K

17.68 g Cr

--------- = 0.3400 mol

52.00 g/mol Cr

19.04 g O

--------- = 1.1900 mol

16.00 g/mol O

Now we have direct numbers in regards to a molecule by molecule basis with which we can compare them. The next step is to normalize these numbers by dividing them all by the smallest of the values in hopes of getting whole numbers which are much easier to use in ratios.

0.3400

------ = 1 K and 1 Cr

0.3400

1.1900

------ = 3.5 O

0.3400

This means that we have 1 potasium, for every one chromium, and 3.5 oxygens.

1 K

1 Cr

3.5 O

Molecular formulas are always based on whole numbers so we can now double all these numbers to find a real ratio

K2Cr2O7

Which is the real answer...

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