chemistry
posted by Tam .
Lab on Determination of copper in brass. I have mass of brass at .1011g. My concentration of diluted solution's unknown is .0037957, My concentration of mg/ml Cu^2+ unknown is 241.2. My professor is asking me to determine the percent of copper. I know that %Cu=mass of Cu/Mass of brass. I know the mass of the brass, I'm not sure how to figure the mass of copper though

If the concn of the Cu^2+ is 241.2 mg/mL, my first question is mL of WHAT? If you know how many mL that is, then 241.2 x mL = total mg, convert that to grams and substitute into your % equation. That will do it.
The GENERAL method for any unknown, but specifically a Cu unknown is the following:
1. Write and balance the equation(s). You may have placed the unknown in solution, added iodate, and titrated the liberated iodine with thiosulfate using starch as the indicator.
2. millimoles standard soln = mL x M
3. Convert mmoles standard soln to mmoles of Cu in sample using the coefficients in the balanced equation(s).
4. Convert to moles Cu and moles Cu x atomic mass Cu = grams Cu
5. (g Cu/g sample)*100 = %Cu