general chemistry
posted by Rosie on .
A student needs to determine by titration with NaOH the precise %KHP in an unknown sample that is thought to contain approximately 50%KHP. Approximately what mass of sample should the student use in order to use about 20mL of 0.1005 M NaOH to reach the endpoint of the titration?
please help me.

First, determine the approximate number of grams of KHP.
moles = M x L = 0.1005M x 0.020 L = ?? moles NaOH.
moles KHP = moles NaOH.
grams KHP = moles KHP x molar mass KHP = xx grams KHP.
%KHP = (grams KHP/mass sample)*100 = 50
You know grams from the calculations above, substitute into the percent equation; the only unknown is mass sample. [Note: In the real world, especially where an analyst does many titrations in a day, the mass of sample is actually weighed out (to four places) so that the reading on the buret corresponds to the percent X in the sample (done for other titrations in addition to acidity). Saves a lot of time on calculations. Read the buret and write down the percent in the lab book. This coupled with automatic fill and zero burets saves even more time. And in industry, time is money.] 
thank you very much for your help!

0.286

.822
When rearranged, its
.411g KHP x 100
 = mass
50
Which equals .822