You are Here: Home >< A-levels

# chemistry as question help watch

1. Not sure how to work this out even though it's only worth one mark.
Bromine has two isotopes BR-79 and Br-81 the relative atomic mass of br is 79.9 calculate the percentage of br-79 atoms in a sample of bromine.
2. 79.9 = (a)79 + (b)81
a+b = 1 (as we're talking percentages)
Therefore a = 1-b
79.9 = (1-b)79 + (b)81
79.9 = 79 -79b + 81b
79.9 = 79 + 2b
0.9 = 2b
b = 0.45
Therefore a = 0.55
79.9 = 0.55*79 + 0.45*81

55% atoms are BR-79, 45% BR-81
79.9 = (a)79 + (b)81
a+b = 1 (as we're talking percentages)
Therefore a = 1-b
79.9 = (1-b)79 + (b)81
79.9 = 79 -79b + 81b
79.9 = 79 + 2b
0.9 = 2b
b = 0.45
Therefore a = 0.55
79.9 = 0.55*79 + 0.45*81

55% atoms are BR-79, 45% BR-81
you're a star, that's a lot for 1 mark.
4. (Original post by medicaesthetic)
you're a star, that's a lot for 1 mark.
It's not too much - just algebra. I showed you all the steps, just to be 100% that you didn't get confused.

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: May 21, 2016
Today on TSR

### Three reasons you may feel demotivated right now

...and how to stay positive

### Can I get A*s if I start revising now?

Discussions on TSR

• Latest
Poll

## All the essentials

### Student life: what to expect

What it's really like going to uni

### Essay expert

Learn to write like a pro with our ultimate essay guide.

### Create a study plan

Get your head around what you need to do and when with the study planner tool.

### Resources by subject

Everything from mind maps to class notes.

### Study tips from A* students

Students who got top grades in their A-levels share their secrets

## Groups associated with this forum:

View associated groups
Discussions on TSR

• Latest

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