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Offline inazir

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Dilution calculation
« on: November 11, 2015, 11:58:34 AM »
Dear All,

I'm doing a dilution calculation based on the following information.
Initial concentration of HMD (aq) 90% (wt). Final  concentration of HMD  40% wt.
I've followed the following steps to calculate the volume of water required for dilution.

1. Calculate the molarity of HMD at 90% wt concentration
Molarity = (10*conc%(wt)*density) / molar mass
HMD molar mass 116.207 g / mol
HMD density 860 kg / m3 @ 90% and 60 degC
HMD 90 % conc wt
Molarity = 6.66 M (mol / litre)
 
2. Calculate the amount of HMD required to make 90% conc wt.
HMD required = molarity * molar mass * volume (litre) = 774 g

3. Calculate the molarity of HMD at 40% wt concentration
Using the same density as step 1
Molarity = 2.96 M (mol / litre)

4. Calculate volume of water required
C1*V1 = C2*V2
V1 = 1 litre
C1 = 90%
C2 = 40%
V2 = 2.25 litres
Volume of water required 1.25 litres

Different approach, repeat step 3 and 4.

3a. Calculate the molarity of HMD at 40% wt concentration
Now using HMD density 923.6 kg / m3 @ 40% wt conc and 60 degC
Molarity = 3.18 M (mol / litre)

4a. Calculate volume of water required
M1*V1 = M2*V2
M1 = 6.66 M
M2 = 3.18 M
V1 = 1 litre
V2 = 2.1 litres
Volume of water required 1.1 litres

If I use the density of HMD at 860 kg /m3 in step 4a and the molarity of step 3 i.e 2.96 M, the calculated volume of water required is 1.25 litres, i.e. the same as step 4.

I'm not sure why the answers are different for working out the amount of water required. Obviously i'm doing something wrong.

Can someone shed some light on this?


Thanks,
Imran

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« Last Edit: November 12, 2015, 05:38:19 AM by Arkcon »

Offline Borek

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Re: Dilution calculation
« Reply #1 on: November 12, 2015, 05:02:10 AM »
Using the same density as step 1

You can't use the same density here, it has a different value.
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Offline inazir

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Dilution Calculation updated
« Reply #2 on: November 15, 2015, 02:17:48 PM »
Hi guys,

I'm doing a dilution calculation and I need to work out how much water to add to get the required HMD concentration.

HMD (aq) at 90% (wt) concentration is diluted to 40% (wt) concentration at 60 degC. 

1. Calculate the molarity of HMD at 90% wt concentration

Molarity = (10*conc%(wt)*density) / molar mass
HMD molar mass 116.207 g / mol
HMD density 860 kg / m3 @ 90% and 60 degC
HMD 90 % conc wt
Molarity = 6.66 M (mol / litre)

2. Calculate the amount of HMD required to make 90% conc wt.
HMD required = molarity * molar mass * volume (litre) = 774 g

3. Calculate the molarity of HMD at 40% wt concentration
HMD density 923.6 kg / m3 @ 40% and 60 degC
Molarity = 3.18 M (mol / litre)

4. Calculate volume of water required
C1*V1 = C2*V2
V1 = 1 litre
C1 = 90%
C2 = 40%
therefore V2 = 2.25 litres, total solution volume

Is the difference of V2 - V1, the volume of water required i.e. 1.25 litre?

Alternatively

4a. Calculate the volume of water required

M1*V1 = M2*V2
M1 = 6.66 M
M2 = 3.18 M
V1 = 1 litre
therefore V2 = 2.1 litres, total solution volume

Is the difference of V2 - V1, the volume of water required i.e. 1.1 litre?

I was expecting the same result using either equation C1*V1=C2*V2 OR  M1*V1=M2*V2
Obviously I'm doing something wrong and I was hoping that someone in this forum could help.

Regards,
Imran
« Last Edit: November 15, 2015, 02:49:04 PM by Arkcon »

Offline Borek

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Re: Dilution calculation
« Reply #3 on: November 16, 2015, 03:44:45 AM »
First of all - why do you convert everything to molar concentrations?

Say you start with a 100 g of a 90% w/w HMD. That means you have 90 g of HMD and 10 g of water.

The only thing to calculate is what mass of water you have to add to get the concentration down to 40%. No need for conversion to molarity, this is a simple application of the w/w % definition.

C1*V1 = C2*V2
V1 = 1 litre
C1 = 90%
C2 = 40%

This equation doesn't apply to w/w % concentrations. It would be OK if you were using solution masses, not solution volumes.
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