# So easy but yet I can't solve it

1. Jan 27, 2014

### cpatel23

Hello.
I am doing a chemistry problem but it is mainly mathematics

Here it is.

A little info that is useful to this problem.

Molar mass is always an average, given by

Molar Mass = (mass of isotope1 * abundance of isotope 1) + (mass of isotope2 + abundance of isotope 2)

Masses of isotopes are always constant, they do not vary from place to place.

So how do I solve this?

2. Jan 27, 2014

### Simon Bridge

While the molar masses do not change, the relative abundances can and do.

An element has isotopes X and Y which have molar masses mx and my with percentage abundances px and py ... rearrange those variables to give an equation for the average molar mass for the element.

3. Jan 30, 2014

### HallsofIvy

Staff Emeritus
If we are to assume that the two isotopes given here are the only ones we need consider, then let "x" be the relative abundance of the first (U235) and then the relative abundance of the other (U238) is 1- x.

So we must have x(235.043924)+ (1- x)(238.050784)= 238.0375.

Solve that equation for x.