You are Here: Home

# Find the mass of something needed to make x g of something else watch

1. Hey, In chemistry, how do i work out this type of question?

For example, the mass of magnesium needed to make 20g of magnesium oxide?

Could somebody talk me through the process of working it out?

Thank you.
2. (Original post by SW1988)
Hey, In chemistry, how do i work out this type of question?

For example, the mass of magnesium needed to make 20g of magnesium oxide?

Could somebody talk me through the process of working it out?

Thank you.
First you need to look at the stoichometric equation:

2Mg + O2 > 2MgO

The ratio of magnesium to magnesium oxide is 2:2, so this means one mole of magnesium will make one mole of magnesium oxide.

You need to find out how many moles of magnesium oxide there are in 20g, then use the above fact to work out how many moles of magnesium you must have used to produce this.
After that, you can find the mass of magnesium when you know how many moles of it there were, and its Mr.
3. 20g / (24 + 16) 40
= 0.5
0.5 x 24 = 12
12g?
4. (Original post by SW1988)
20g / (24 + 16) 40
= 0.5
0.5 x 24 = 12
12g?
That looks good!
5. (Original post by pineneedles)
That looks good!

Thank you, so just to make sure I've got the way of doing it.

The mass of lithium oxide made from 20g of lithium carbonate.

20g/73.89 = 0.27
0.27 x 29.88 = 8.08
8g

20g / 73.89
= 0.27
0.27 x 29.88 = 8.08
8.08g

I don't think this is right.
6. (Original post by SW1988)
Thank you, so just to make sure I've got the way of doing it.

The mass of lithium oxide made from 20g of lithium carbonate.

20g/73.89 = 0.27
0.27 x 29.88 = 8.08
8g

20g / 73.89
= 0.27
0.27 x 29.88 = 8.08
8.08g

I don't think this is right.
That looks fine. What is it that you're not happy about?
I don't know how your exam board works, but I'm taught to write all calculations to three significant figures, so you'd write 8.10 g. Good habits and all that.
7. (Original post by pineneedles)
That looks fine. What is it that you're not happy about?
I don't know how your exam board works, but I'm taught to write all calculations to three significant figures, so you'd write 8.10 g. Good habits and all that.
My problem is I'm just not that confident with it yet, so I'm doubting myself with everything. Thank you for your help, I ran both examples by my private chemistry tutor this evening and he says the way I've done it is right, which you agree with. I will use three significant figures from now on, too!
8. (Original post by SW1988)
My problem is I'm just not that confident with it yet, so I'm doubting myself with everything. Thank you for your help, I ran both examples by my private chemistry tutor this evening and he says the way I've done it is right, which you agree with. I will use three significant figures from now on, too!
You're very welcome Confidence will come with practise, as you probably know!
9. (Original post by pineneedles)
You're very welcome Confidence will come with practise, as you probably know!
Yeah, definitely! I just need to trust myself. I'll definitely get confident with practice, for half-term I've got a huge booklet of mole calculations/redox/empirical formual questions to do.
10. Dear Student:
Iam a Certified Teacher with extensive experience teaching Chemistry (graduateand specialist) as well as in chemical research (Dr. in chemistry). I also workas a graduate thesis advisor in the area of chemical research and teaching. Iam author and referee of indexed scientific articles.Inmy spare-time I work answering specific questions by email (English andSpanish) or Skype (Spanish only). In this sense, if you need explanatoryanswers to some questions I can help you with pleasure. If you are interestedin my work method, for today or other day, you can contact me by email:freelanceprojects3.0 gmail.comBestRegards
QSAE-Education,Science & Solution
Iam a Certified Teacher with extensive experience teaching Chemistry (graduateand specialist) as well as in chemical research (Dr. in chemistry). I also workas a graduate thesis advisor in the area of chemical research and teaching. Iam author and referee of indexed scientific articles.Inmy spare-time I work answering specific questions by email (English andSpanish) or Skype (Spanish only). In this sense, if you need explanatoryanswers to some questions I can help you with pleasure. If you are interestedin my work method, for today or other day, you can contact me by email:freelanceprojects3.0 gmail.comBestRegards
QSAE-Education,Science & Solution
(Original post by pineneedles)
First you need to look at the stoichometric equation:

2Mg + O2 > 2MgO

The ratio of magnesium to magnesium oxide is 2:2, so this means one mole of magnesium will make one mole of magnesium oxide.

You need to find out how many moles of magnesium oxide there are in 20g, then use the above fact to work out how many moles of magnesium you must have used to produce this.
After that, you can find the mass of magnesium when you know how many moles of it there were, and its Mr.
12. Thank you but no thanks.
13. I have a new question but don't want to start a new thread.

What mass of sodium carbonate should I add to a 20cm3 solution of 0.5 mol/dm3 nitric acid to neutralise it?

### Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: November 22, 2015
Today on TSR

### Exam Jam 2018

Join thousands of students this half term

### Solo - A Star Wars Story

Poll

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE