Oh chemistry, oh chemistry, how horrible are your questions..(think the "oh christmas tree" tune as you read )

In a routine laboratory analysis, a solution of Potassium Chromate with a Chromium concentration of 40.0 ppm is required. Calculate the molarity of this solution.

okay, so this is for my Chemistry Lab. I'm in college and studying to be a Chiropractor. We covered molarity in the lecture class, and i was rather skilled at it (if i do say so myself). However, I have never run across a question like this involving "ppm." I'm not exactly sure what to do with it...could you please help me?

40ppm = 40mg / litre
potassium chromate = K2CrO4
What mass of K2CrO4 will contain40mg of Cr
Molar mass K2CrO4 = 194.1905 g/mol
Molar mass Cr = 51.996g/mol
Mass K2CrO4 = 194.1905/51.996*40 = 149.39 mg

Therefore you require 149.39mg OR 0.1494g K2CrO4 in 1 litre of solution
How many mol is this
mol = 0.1494/194.1905 = 0.00077 mol in 1 litre of solution
Molarity = 0.00077M

40ppm is 40 parts per million ie 40 grams in 1 million grams of water = 40 gms in 1000 litres of water = 0.04 gms per litre

One molar solution would contain the AW of chromium per litre - ie 52 gms per litre. (never mind the AW of the other elements in K2CrO3 - you're told the Cr concentration)

But you have only 0.04 - ie 0.04/52 times less => solution is 0.00077 molar