Question

Note:

1 mole of a compound is equivalent to its molecular mass

1 mole of a compound is equal to 6.02x10^23 molecules

The percentage is a direct equivalent to grams

Answers should be in 3 significant figures

No need to include the units and the compound in the answers

1. A compound is found to contain 63.52 % iron and 36.48% sulfur. Find its empirical formula. 
2.   A compound is found to contain 26.56% potassium, 35.41% chromium, and the remainder oxygen. Find its empirical formula.  

 

  

Answer 1

1. One gram of a compound contain 0.6352 g iron and 0.3648 g sulfur.

$n(Fe) = \frac{0.6352}{55.84} = 0.011375 \; mol \\ n(S) = \frac{0.3648}{32.06} = 0.011378 \; mol$

Proportion:

n(Fe) : n(S) = 0.011375 : 0.011378 = 1:1

Empirical formula: FeS

2. One gram of a compound contain 0.2656 g potassium and 0.3541 g chromium, and 0.3803 g oxygen.

$n(K) = \frac{0.2656}{39.10}=0.006792 \;mol \\ n(Cr) = \frac{0.3541}{51.99}= 0.006810 \;mol \\ n(O) = \frac{0.3803}{15.99}= 0.023783 \;mol$

Proportion:

n(K) : n(Cr) : n(O) = 0.006792 : 0.006810 : 0.023783

$= \frac{0.006792}{0.006792} : \frac{0.006810}{0.006792} : \frac{0.023783}{0.006792} \\ = 1 : 1 : 3.5 = 2: 2: 7$

Empirical formula: K₂Cr₂O₇