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# A2 Chemisry watch

1. Hi! I just have a quick question for chemistry.
I am lost on whhy when you dilute a solution the concentration becomes less but moles stay the same, but when you take lets say half of a solution moles become half yet concentration stays the same....

Thankk you!!
2. when you dilute a solution you are increasing the volume but the moles stay the same

And how do you work out concentration?

concentration = moles / volume

so if you dilute a solution the new conc. is same number of moles/ larger volume , i.e the same number divided by a larger number- which of course will give you a smaller number (hence the concentration decreases)

Hope this attempt at an explanation helps lol
3. (Original post by ranz)
Hi! I just have a quick question for chemistry.
I am lost on whhy when you dilute a solution the concentration becomes less but moles stay the same, but when you take lets say half of a solution moles become half yet concentration stays the same....

Thankk you!!
because concentration = moles of the substance/ total volume
so if you increase the volume that it's in (by diluting it) but keep the moles the same (so don't change the amount of the substance itself you have in there), your concentration will decrease

lets say you had a bag with 6 red marbles in it. This is very concentrated because it just has red marbles in it but if you add 10 blue marbles to the bag you'll STILL have 6 red marbles (same moles) but there's blue ones in there as well so it is less concentrated now

also same equation, moles = volume x concentration
so if you decrease the volume, but keep concentration the same, your moles will also decrease
say you had the red bag again with the 6 red marbles and 10 blue marbles - the ratio is 3:5
conc = moles/volume = 6/16 = 0.375

if you took 8 marbles out of that bag
you'll have 3 red marbles and 5 blue marbles
so HALF the number of moles but the ratio is still 3:5
conc = moles/volume = 3/8 = 0.375

same concentration! different moles

i hope that made sense (and was correct!)
4. Moles are to do with the number of particles/molecules in a solution. Pretend you have 1 litre of solution in a jug. In it you have one mole of OH- ions. (One mole of any element is equal to 6.022x10 ^22 according to Avogadro so you have 6x10 ^22 ions in that solution).

If you add 0.5 litres of another solution to the jug, you've diluted it as you've made the volume of the overall solution bigger, but the number of OH- ions still remain the same. The number of moles can be found be using the equation: number of moles = concentration x volume.

If the number of moles is staying the same but the volume is increasing, the concentration of the solution must decrease for the formula to still work. This make sense?

Remember to think of moles as number of 'things' and it should help distinguish between concentration and moles. If you have five pens, 3 blue and 2 red. If we added 5 more blue pens, we'd still have the same number of red pens (moles stays the same) but now it takes up a smaller percentage of the group of pens (concentration has decreased).

Hope that helps.

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Updated: January 19, 2016
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