[positiveion]

When 250 cm^3 of 3.00 mol dm^-3 HCl is added to 350 cm^3 of 2.00 mol dm^-3 Hcl the concentration of the solution of hydrochloric acid obtained in mol dm^-3 is:

This is my working:

250 cm^3 of 3.00 mol dm^-3 HCl:
250 ml of .3 mol per 100 ml
THEREFORE: 2.5*.3= .75

350 cm^3 of 2.00 mol dm^-3 HCl:
350ml of .2 mol per 100 mol
THEREFORE: 3.5*.2= .7

.75+.7 = 1.45

Therefore I thought the answer would be 1.45 mol dm^-3

HOWEVER, the answer is 2.42 mol dm^-3

and I have no idea why.

[Borek]

You know there will be 1.45 mole of HCl, but mole is not a concentration unit.

[positiveion]

Oh - how do you figure out the concentration, then?

[sjb]

Oh - how do you figure out the concentration, then?

What is the definition of concentration?

[positiveion]

density

so its
density = mass/volume
density = 1.45/600
= 0.00241666666

... ?

[sjb]

density

Why did you not use the concentration figures in the first place in the same way then?

...
250 cm^3 of 3.00 mol dm^-3 HCl:
250 ml of .3 mol per 100 ml
THEREFORE: 2.5*.3= .75
...

[positiveion]

I'm not sure what you mean would you mind just explaining it to me?

I tried doing it like

.75/250 = .003
.7/350 = .002

.003+.002=.005 which is the wrong answer ...

[Borek]

You were already asked that question: what is concentration definition? If you don't know, check in your textbook, notes, whatever.

[positiveion]

I already said that the definition of concentration is density and I tried plugging numbers into it but since I got the wrong answer I must have done something wrong. I don't know what - that's why I'm asking. If I could figure it out from my textbook I would.

[DrCMS]

I already said that the definition of concentration is density

No NO  NO NO it is not, concentration is one thing and density is another. 
If you do not know something look it up don't just guess.

Density is mass per unit volume
Molarity is moles per litre

You should be working with the 2nd equation.

If you have 0.5 moles in 1L and add it to 0.5 moles in 2L the total no of moles is 1 but as you know have 3 litres of solution the molarity is 0.33 moles per litre.

[positiveion]

I DID look it up! And virtually the first result I got was a website saying that concentration is density..

[positiveion]

Great! I got it then!

1.45 moles/600cm^3

600 * X = 1000
X = 1000/600
X = 5/3

1.45 * 5/3 = 2.41666666667

Therefore:
2.42 mol dm^-3

Thank you!!

[DrCMS]

I DID look it up! And virtually the first result I got was a website saying that concentration is density..

Well don't use that website again because it talks crap.

Great! I got it then!

1.45 moles/600cm^3

600 * X = 1000
X = 1000/600
X = 5/3

1.45 * 5/3 = 2.41666666667

Therefore:
2.42 mol dm^-3

Thank you!!

Yes that's it you have 1.45 moles in 600ml which is 1.45x600/1000 = 2.42 moles per litre.

[Borek]

I DID look it up! And virtually the first result I got was a website saying that concentration is density..

Well don't use that website again because it talks crap.

Or he misunderstood it

Positiveion, care to post the link?