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

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.

20g / (24 + 16) 40
= 0.5
0.5 x 24 = 12
12g?

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.

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!

You're very welcome Confidence will come with practise, as you probably know!

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.