One of the most common errors in have observed for both GCE A-Level H2 Chemistry (as well as 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 A-Level H2 Chemistry Weekly Tuition Class:
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.
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 C3H10N2.
Hope the above helps you in clarifying some of your doubts.
PS: Let me know how you find about this post on Common Errors in Chemistry. I would love to hear from you. =)