Really struggling with mole calculations, just can't seem to get my head around them. any advice?

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Been working at these for a while now and getting no-where, it seems to make sense when it's explain but as soon as i try out a question i always get the wrong answer and i can't see where i have gone wrong! 

Posted Tue 2nd April, 2013 @ 15:43 by Fraser
Edited by Fraser on Tue 2nd April, 2013 @ 15:43

1 Answer

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Do you use mole triangles? They really helped when I began the calculations. If you are working out the molarity of a solid...

Draw a triangle. At the top of the triangle, put mass (in grams). At the base, in each corner, put Mr (relative molecular mass) and Moles. The triangle works on the principle that if you cover what you are trying to investigate, the calculation is revealed.

   Mass
__________

Mr   x   Mol

E.g... 4.3g of Fe, what is the molarity? Cover up Moles to reveal Mass / Mr.
4.3 / 55.8 = 0.07 moles.

Similarly, you can find the mass if you know the moles - 55.8 x 0.07 = 4.3g

If you are finding out molarity of solutions, it gets a bit more tricky. Same principle, but moles on top and concentration (in moldm-3) and volume (in dm3) on the base.

All the best.

Answered Tue 2nd April, 2013 @ 20:21 by Didgez