# Chemical Forums

## Chemistry Forums for Students => High School Chemistry Forum => Topic started by: DarkS on October 14, 2020, 11:56:38 AM

Title: Help, Questions on finding Volume from concentration.
Post by: DarkS on October 14, 2020, 11:56:38 AM
During a chemistry lesson, almost all of the students got the answer for a question wrong. When the lesson had ended the teacher hadn't yet told us the problem with our method and most of us are quite passionate to find the correct answer now.

Mass of solute: 12g of MgCl2
The volume of liquid:?
Concentration mol/dm3: 1.5 mol/dm3
Concentration g/dm3: 19.6/dm3

In a previous lesson, we had learnt that:

Concentration mol/dm3 = number of moles / volume (dm3)

so rearranging this gives:

Volume = number of moles / Concentration

To work out the number of moles I had taken the mass (12g) and divided it by its relative formula mass (95):

Number of moles = 12/95 = 0.12631578947

then I substituted it into the earlier equation: Volume = number of moles / Concentration

Volume = 0.126 / 1.5 = 0.0084dm3

However, the teacher's answer was 8dm3, though I'm pretty sure my answer is wrong I don't know where I went wrong. All help is appreciated

I've attached the worksheet as pdf where the question is from but it has my messy writing over some of it so it may be unclear.
Title: Re: Help, Questions on finding Volume from concentration.
Post by: chenbeier on October 14, 2020, 12:38:47 PM
Neither 0,0084 dm3 nor 8 dm3 is correct.
The  result is 0,084 dm3

Die teachers value is to large yours to small.

Also die concentration 19.6 g/ dm3 is wrong.

1.5 mol/dm3 x 95 g/mol = 142,5 g/dm3

Title: Re: Help, Questions on finding Volume from concentration.
Post by: DarkS on October 14, 2020, 01:27:38 PM
Thanks, it seems I made a careless calculating mistake. I will wait for my teacher's explanation on how she got 8 to see her points.