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*:

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 C_{2}H_{5}N.

– 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)3102As such, my suggested answer for the empirical formulae will be C

_{3}H_{10}N_{2}

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 C_{2}H_{5}N. 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 C_{3}H_{10}N_{2.}

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. =)

jauhar zubair kisambira says

I have also done the same mistake like one of your students as i was trying it out but i hope i will rectify it follwing your suggested answer.

However why are you multiplying it the the simplest ratio by two if in our lessons in o leval we were told to round off to the nearest whole number ?……………….

lastly but not least, i thank you for your suvice and please provide more if possible.

zain says

i got the right answer. thankx for the post.

Sean says

When the no. of moles is calculated to be close to an integer (full number), then we can usually round it up or down…

E.g. 1.97 you can safely say it’s 2.

E.g. 0.986 you can safely say it’s 1.

because the error is very small.

In fact, even in my GCE O-Level Pure Chemistry Tuition Classes, we teach them the proper way. i.e. any calculated value is not near to an integer, we must do the above.

Hope this is clear to everyone.

PS: If you find this useful, feel free to share it with your friends.