Homework Help: chem

Posted by Tricia on Friday, December 7, 2012 at 8:55pm.

A 0.050 L solution of H2SO4 requires 200.0 mL of 4.0 M NaOH for phenophthalein to turns pink to colorless. What is the molarity of the original H2SO4 solution? I got 4M but my instructor said it's 8M how did he get that? Pls need help figuring it out. Thanks

chem - DrBob222, Friday, December 7, 2012 at 10:10pm
H2SO4 + 2NaOH ==> Na2SO4 + 2H2O
mols NaOH = M x L = 4.0M x 0.200L = 0.8 mols.
Use the coefficients in the balanced equation to convert 0.8 mol NaOH to mols H2SO4. That will be 0.8 mol NaOH x (1 mol H2SO4/2 mol NaOH) = 0.4 mol H2SO4.
Then M H2SO4 = mols H2SO4/L H2SO4 = 0.4/0.050L = 8.0M

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