Eli5: what is exactly is molarity.


if you have 2 solutions for example
50ml of 0.5M of a solution (A)
and 0.25M of 100ml of a solution (B)
Would the new volume be 150ml or 0.150L
And would I have to change the molarity of solutions A and B

In: 2

Yes, both substances would be diluted to a lower concentration when combined.

B would now be 0.025 moles in 150ml and A would also be 0.025 moles in 150ml. (.1666 M for both components)

> Would the new volume be 150ml or 0.150L

Those are the same value.

> And would I have to change the molarity of solutions A and B

The molarity of the mixed solution is different from the molarity of solutions A and B, if that’s what you’re asking.

Remember what molarity means: it means the number of moles of whatever the dissolved substance is, divided by the number of liters it’s dissolved in. To find the molarity of the mixed solution, figure out how many moles of the substance are in the mixture (hint: it’s the number of moles of the substance that were in mixtures A and B, added together – the *molarities* don’t add, but the *number of moles* does).

I read the title as “what exactly is *morality*” and got very confused when you started talking about ml and solutions.

However! I think I can still answer your question.

If you combined the two solutions then yes you would get 150ml of solution. However the molarity would be different. To calculate the molarity of the combined solution you need to know what molarity means.

Molarity is basically the number of moles per litre. So what you need to do is calculate the number of moles in A and the number of moles in B and that gives you the number of moles in the combined solution. Divide that by the number of litres in your final solution and that will bring it back to a molarity measurement.

Solution A has a molarity of 0.5M and there’s 50ml of it. So that means there are 0.025 moles in solution A (`0.5M X 0.05L`)

Solution B has 0.25M in 100ml so there are 0.025 moles in solution B

So in the combined solution there are 0.05 moles. And the solution volume is 150ml. So the molarity of the new solution is `0.05/0.15` which is a third (0.3333…M).

At least that’s what I calculated it as…happy for people to point out my mistakes!

Edit: I’ve assumed above (probably incorrectly) that the two solutions are of the same thing. If they aren’t of the same thing then you can’t combine their moles so you would have 0.025 moles in 150ml (0.1666 M) of each component

I just wanna say that those of you who understand this are amazing. I’m a pretty educated person but this is like a foreign language to me. You should be proud of yourselves.

Polarity = the opposing groups of polar bears.

Molarity = the opposing factions of mole people.

People get so confused whenever moles come into it. The important thing is to remember that it’s just another number, like “dozen” or “couple”. In chemistry’s context, it’s the number of atoms or molecules of a substance. And “molarity” is just “number of atoms/molecules per volume”.