# Naked Science Forum

## Non Life Sciences => Chemistry => Topic started by: Indranil on 11/08/2018 01:59:36

Title: How do you calculate the number of atoms in something based on percentage mass?
Post by: Indranil on 11/08/2018 01:59:36
Two oxides of a metal contain 27.6% and 30% of oxygen respectively. If the formula of the first oxide is MO, find that of the second?
Now my problem is how to find the atoms of O from the masses of 27.6% and 30.0 %? I am confused.
Title: Re: How do you calculate the number of atoms in something based on percentage mass?
Post by: chiralSPO on 17/08/2018 22:18:42
There are two ways I see to approach this problem:

One is to choose an arbitrary amount of substance and calculate how many moles of each of the atoms there are. For instance, we can ask how many moles of O there are in 100 g of the compound, given that 27.6% of it is O (and then because it is stated, we also know that this value is also the number of M atoms).

The other way is to forget actual numbers of atoms (or moles, etc.) and just deal with the ratios (this is like the question you posted earlier, about volumes of gases--we don't need to say what volume it is, just compare the ratios).

Hopefully that is enough to get you started.
Title: Re: How do you calculate the number of atoms in something based on percentage mass?
Post by: Indranil on 18/08/2018 02:24:29
Two oxides of a metal contain 27.6% and 30% of oxygen respectively. If the formula of the first oxide is MO, find that of the second?
Now my problem is how to find the atoms of O from the masses of 27.6% and 30.0 %? I am confused.

Could you elaborate the two methods a little bit so that I could start?
Title: Re: How do you calculate the number of atoms in something based on percentage mass?
Post by: chiralSPO on 18/08/2018 13:31:26
 To get started for method 1: If you have 100 g of a compound that is 27.6% O by mass, how many grams of O is there in the sample? Given the atomic mass of O, how many moles is that equal to?

 To get started for method 2: If the compound MO is 27.6% O by mass, what is the atomic mass of M?