You are Here: Home >< A-levels

# AS Chemistry Help watch

1. 1) What does weighted mean in this statement? : Relative atomic mass of an element is the weighted average atomic mass of the element relative to 1/12 the mass of an atom of the C12 isotope.

2) Why do we use 1/12 the mass of an atom of the C12 isotope? (My teacher said its because it is thought to be stable)

3) What does the relative atomic mass of chlorine (35.5) tell you about the relative abundance of the 2 naturally occurring isotopes of chlorine 37 and chlorine 35?
2. bump
3. pls
4. This is very simple GCSE knowledge, and simple maths.

1) Weighted - depending on the abundance of certain isotopes in the world, the atomic mass is calculated as an average.
It's the same maths used when calculating a mean in a frequency table in maths.

2) Carbon-12 has a stable arrangement (6 protons, 6 neutrons, 6 electrons), is relatively common in the world, and is a solid at a normal temperature, so is easy to store and transport. So, it's convenient to use Carbon.

3) 35.5 tells you that 75% of the world's Chlorine is Chlorine-35, and 25% of the world's Chlorine is Chlorine-37. The formula to work this out is = (Mass x Abundance) + (Mass x Abundance) ... (in this question there are only 2 isotopes), then divide that by the total abundance (usually 100%), giving you 35.5
5. 1) weighted means its been compared to other isotopes of the same element and its abundances - to get an average mass for an atom of the element.

2) Its because its Ar is "stable" (its Ar is a whole number - which is 12 so its easier to use to compare with other atoms because other atoms have an Ar with lots of decimals/ are odd numbers.)

3) It tells us that theres 75% abundance of cl-35 and 25% abundance of cl-37 ((75x35)+(35x25) divide by 100 = 35.5)
6. AnnaRainbows Ishan_2000 i didnt learn anything about abundance in year 10/11 literally 0, this is all new to me. we learnt basics of isotopes and how they differ but thats it. thanks ishan for helping and shohaib712
7. (Original post by sfaraj)
AnnaRainbows Ishan_2000 i didnt learn anything about abundance in year 10/11 literally 0, this is all new to me. we learnt basics of isotopes and how they differ but thats it. thanks ishan for helping and shohaib712
Oh, ok, that's a bit weird. Anyway, you have learnt it now.

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: September 12, 2016
Today on TSR

### The most controversial member on TSR?

Who do you think it is...

### Uni strikes! How do they affect you?

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