Question

A substance was analysed and found to contain 26.57 % potassium, 35.36% chromium, and
38.07 % oxygen by mass.
What is the empirical formula of the substance?

Select one:
question
O a. K2Cr205
O b. K2Cr207
O c. KCr2O7
O d. K_Cro

Answer 1

Answer: K2Cr2O7

Explanation:  Pretend we have 100g of the compound.  use the percentage to calculate how many grams of each element are in the 100g.

K:  26.57g

Cr:  35.36 g

O:  38.07 g

Now divide by the molar mass of each element to find the moles of each:

K:  0.7 moles

Cr:  0.7 moles

O:  2.4 moles

That gives us 3.8 moles total.  But what's important is to find a factor that would bring all of these mole values to a whole number, since they must be in whole number proportions.  The closest I got was around 3.

That would lead to the following empirical formula:  K2Cr2O7.