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One of the most common errors in have observed for GCE O-Level Chemistry students is when they are Calculating Empirical Formulae from Composition by Mass.

Take the following question that was given to my students:

Question:

Calculate the empirical formula of a compound that has the composition: 48.8% carbon, 13.5% hydrogen and 37.7% nitrogen.

After calculations, one of my students came up confidently with the answer of C₂H₅N.

- which is the Incorrect Answer.

After asking her to present her working to the class, i we realised that she made a mistake when trying to round off final numbers.

Let’s take a look at my suggested answer & then see how she made the mistake – which is a Common Error for many Chemistry students year-after-year.

Suggested Answer:

Element	C	H	N
Mass	48.8	13.5	37.7
Ar	12	1	14
Mole	48.8 / 12 = 4.07	13.5 / 1 = 13.5	37.7 / 14 = 2.69
Molar Ratio	4.07 / 2.69 = 1.51	13.5 / 2.69 = 5.02	2.69 / 2.69 = 1
Simplest Ratio (x 2)	3	10	2

As such, my suggested answer for the empirical formulae will be C₃H₁₀N₂

However, my student insists argues that when we get the Molar Ratio of 1.51 – it is more than the half way mark of 1.5 and we should round it up to 2. As such, her answer will be C₂H₅N. Do note that this is Incorrect!

The correct strategy is to try to get rid of the fraction (1.51 ~ 3/2) and in this case, multiplying throughout by a factor of X2 will solve the problem and give us the correct empirical formula of C₃H₁₀N_2.

Hope the above helps you in clarifying some of your doubts.

PS: Let me know how you find about this post on Common Chemistry Errors. I would love to hear from you. =)

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• O Level Chemistry: Determining Molecular Formula from Empirical Formula
• O Level Chemistry: Mole Calculation Question & Common Mistakes Made
• O Level Chemistry: Molecular & Empirical Formulae
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• O Level Chemistry: Mole Concepts / Chemical Calculations