Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: rachymc on December 04, 2007, 09:08:22 AM
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Have a problem...
What is the concentration in ppm of 250mL of a .0015M solution of K2Cr2O7 solution?
So far i've got..
no moles = molarity X vol in litres..
So that is (.0015M)(.25L) = .000375moles.
So in grams thats (.000375moles)(294.18g/mole) = .110375grams.
To mg = 110.375mg/250ml
So for ppm is that 110.375ppm? Because ppm is mg/L...so would that mean its 110.375 X 4 = 441.5mg/L which is 441.5ppm?
Im confused... ???
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To be accurate, ppm is simply a ratio and has no dimensions (units of measurement). So, 1 ppm can be 1 gram in a million grams or 1 litre in a million litres or one molecule in a million molecules. Your answer of 441.5ppm is correct. When dealing with aqueous solutions, ppm and mg/l are considered to be the same thing despite the fact that mg/l is a weight per volume. However in aqueous solutions this is ok (ish) since water has a density of 1 and 1 litre will weigh 1,000,000 mg.
When dealing with concentrations of volatile analytes in air, then ppm means something completely different! In this case, 1 ppm means one molecule of analyte in one million molecules of air. So to convert ppm to mg/l, you need to know the analyte's molecular weight and it's molar volume (22.4 litres ish).
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rachmyc: volume doesn't matter, ppm is a measure of concentration and is not volume dependent.
ppm is ill defined and can mean anything. It can be weight/weight, volume/volume or particle/particles.