## Recommended Posts

Hello,

I have got this question i'm having difficulty with... Here it is:

(b) A beaker contains 250cm3 of 0.3 mol dm-3 calcium hydroxide Ca(OH)2 solution.

(i) How many moles of calcium hydroxide are present in 150cm3?
(ii) What mass of calcium hydroxide is present in the solution?

250cm3 in dm3 is 250/1000 = 0.25dm3

0.25 x 0.3 = 0.0075 moles of calcium hydroxide

so... 0.0075mol x 74mr = 0.555g of calcium hydroxide?

##### Share on other sites

I assume there is a typo in b (i) and that you meant to say 250 cm3? If so, then this looks fine to me, except that I think you mucked up your calculation in part (i). If you fix that your numbers should work.

##### Share on other sites

Thanks for the help. I think it is supposed to say 150cm3. I think question b(ii) is asking how much is in the solution stated in (b)

##### Share on other sites
Quote

mol dm-3

This is not clear.

Do you mean mole per deci (metre)3 or mole per (decimetre)3 ?

##### Share on other sites

I have absolutely no idea. it's the assessment question

It says mol dm-3

##### Share on other sites

Well there's a big difference between one tenth of a cubic metre and one tenth of a metre cubed.

dm-3 should be one tenth of a cubic metre or (1 million cubic centimetres)/10 = 100,000 cubic centimetres.

A decimetre cubed = (10cm x 10 cm x 10 cm) = 1000 cubic centimetres. and should really be denoted (dm)-3

But I should ask someone what was actually meant.

Edited by studiot

##### Share on other sites

It’s just moles per litre, it’s perfectly fine and very commonly used. Edited: I kind of see your point about brackets, but I also don’t think it’s at all ambiguous as written.

3 hours ago, Kimistry said:

I have absolutely no idea. it's the assessment question

It says mol dm-3

There’s nothing wrong with the way you wrote it, I don’t think.

3 hours ago, Kimistry said:

Thanks for the help. I think it is supposed to say 150cm3. I think question b(ii) is asking how much is in the solution stated in (b)

Got it. Normally these sorts of questions try to step you through the calculations by asking for each bit separately, so I had thought maybe it should have been 250 mL. Did you see what I meant by the calculation being mucked up in the calculator? I think you were out by a factor of 10.