Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: Compaq on November 09, 2014, 09:07:11 AM
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Hi,
Let's say the ICP-OES software gives me a concentration of 186,49 ppm (mg/L) of Mn. I diluted the acid digested sample to 14.2 mL. I weighed in 10.17 mg of the powder I analyzed. I need to calculate the actual concentration.
It is quite logical in my head what I need to do: multiply by diluted volume and divide by inweight. However, I don't understand my units.
When I multiply 186.49 mg/L with 0.0142 L, I get 22,648 mg. When I then divide this by 10.17 mg, I end up with 0.260. But this is unitless. How do I know what unit I have? I thought I converted the w/V concentration to w/w by this procedure, but the units does not follow.
I am sure there is an easy answer to this question, but I have looked at it for too long to get any wiser!
I would appreciate any help.
Anders
PS I just realized something. If I ended up with a ratio (not sure what kind of ratio), then I think I understand. 0.26 would then represent 26 %, meaning my sample contains 26 % Mn. But how would I get this to an w/w expression, such as mg/kg, or g/kg?
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1) what's the physical dimension of [weight]/[weight]? -> dimensionless!
2) conversion: kg/kg = g/g = mg/mg = 1000 mg/g etc....
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When I multiply 186.49 mg/L with 0.0142 L, I get 22,648 mg.
One '2' too much.
When I then divide this by 10.17 mg, I end up with 0.260. But this is unitless. How do I know what unit I have? I thought I converted the w/V concentration to w/w by this procedure, but the units does not follow.
As Irlanur already wrote, w/w is unitless - so it is just a fraction of the original sample. Multiply by 100% and you will get % unit.