gchem question from TS2

[yumi43]

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hello, i was wondering if someone could explain how to do this problem. the explanation they give on topscore is confusing me. thank you!

"What volume of HCl was added if 20 mL of 1 M NaOH is titrated with 1 M HCl to produce a pH=2?"

the answer is 20.4 mL

 

[baedero1]

0.02 L 1M NaOH --> 0.02 mol
x L 1M HCl --> x mol

pH=2 --> 0.01M HCl

(x mol - 0.02 mol)/(x L + 0.02L) = 0.01

x = 0.0204 L = 20.4ml

 

[yumi43]

ok thank you. i get really confused with titration problems for some reason. would it be possible for you to explain why the equations are set up like that?