AQA A level Chemistry reacting mass calculations

Hello,

So far in my A level chemistry, I have got through the whole of Unit 1. And I wanted to start of with practising mole calculations since this topic is one where I would need to understand it fully in order to successfully provide a correct answer.

But here's where I get to my problem. It's with the rounding. I would usually get an answer that is right but isn't rounded up (At one point I needed to round down, does this count?)

An example of where this happened was on this question:
What mass of potassium oxide is formed when 7.8 mg of potassium is burned with oxygen?

4K+O2->2K2O

I ended up with 0.0093729g when the real answer was 0.00940g. I'm a making a big deal out of nothing or is there a rule on how many significant figures that you use?

Reply 1

TajwarC

Original post by Fruitbasket786

Always use the same number of sig figs as used in the question

Reply 2

Fruitbasket786

Original post by TajwarC

Always use the same number of sig figs as used in the question

That would make plenty of sense, thanks for helping me out :biggrin:

Reply 3

Gaiaphage

Original post by Fruitbasket786

I ended up with 0.0093729g when the real answer was 0.00940g. I'm a making a big deal out of nothing or is there a rule on how many significant figures that you use?

3 sig figs will always get you the mark (unless they specify otherwise) and it's what the markschemes usually use. Same goes for any maths answer in any subject.
The only exception is where the answers are very large and 3 sig figs is just not accurate enough, in these cases you'd just go to 1 or 0 dp. That's unlikely to happen in Chemistry though.

(edited 9 years ago)

Reply 4

Protoxylic

In chemistry, 2sf or more is sufficient. It says it in the mark scheme. However, subjects like physics, you require to quote to the relevant number of significant figures pertaining to the data given in the question. I always quote to 3sf in chemistry, if you quote to 1sf you lose one mark, but I think I am correct in saying you only get penalised once for a significant figure error across the whole paper.

Reply 5

username1445490

Original post by Gaiaphage

This is correct and in exam mark schemes they will accept a range of answers for this type of calculation and your answer would be acceptable unless it said give your answer to 3 sig figs. In this calculation they didn't round during the calculation.

To be correct you shouldn't use more sig figs than the least amount in the data and this question uses 2 sig figs in 7.8 mg but if you apply this rule then the answer if 9.42 x 10-3 g but exam and text book questions are often not written to follow this rule. If you do not round doing the calculation and then round to 3 sig figs (for smaller numbers) at the end you will be fine.

(edited 9 years ago)

Reply 6

rotumanstudent

Hey im new here and there is no other forums on simple covelent molecular substances (discrete)

Im sorry for interepting you guhs but was wonderinv if hou guys could help me...

Why do simple covalent molecules have low melting/boiling point?
And why do like dissolve like?
And why do they not conduct electricity? :yes:

Reply 7

username1445490

Original post by rotumanstudent

You need to start a new thread for your question otherwise it will get mixed up with this one.
Go here http://www.thestudentroom.co.uk/forumdisplay.php?f=130 and click on Post New Thread and ask the question.
Then I am happy to help.

(edited 9 years ago)

Reply 8

rotumanstudent

Original post by Madasahatter