1. What is the molecular formula of a compound that has 85.7% carbon and 14.3% hydrogen by mass, if the molecular mass is 28g/mol?
2. A sample of a compound of boron and hydrogen contains 6.444g of B and 1.803g of H. The molar mass of the compound is about 30g. What is its molecular formula?
3. A mercury salt has the following composition by mass; mercury, 80.69%; Sulfur 6.436%; oxygen 12.87%. Determine the empirical formula of the salt.
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Answer:
C6H12
Explanation:
AS with all these problems, we assume (for simplicity) 100⋅g of unknown compound.
And thus there are 85.7%×100⋅g12.011⋅g⋅mol−1 with respect to carbon, i.e. 7.14⋅mol⋅C
And 14.3%×100⋅g1.00794⋅g⋅mol−1 with respect to hydrogen, i.e. 14.1⋅mol⋅H.
And thus the empirical formula, the simplest whole number ratio that defines constituent elements in a species is CH2.
Now the molecular formula is alway a whole number multiple of the empirical formula:
empirical formula×n = molecular formula
So (12.011⋅g⋅mol−1+2×1.00794⋅g⋅mol−1)×n=84⋅g⋅mol−1.
So n=6, and molecular formula = 6×CH2
= C6H12