It takes 26.8 mL of a 0.0700 M NaOH standard solution to neutralize a 250 mL sample of lactic acid (C3H6O3). What mass of lactic acid was dissolved in the sample?

There is 1 OH - in 1 molecule of NaOH.

Also, there is 1 H + in 1 molecule of lactic acid.

So the reaction is simple.

so just equate the moles

moles of OH - in NaOH = moles of H + in lactic acid

26.8 x 0.07 = 250 x Mole of lactic

Moles of lactic = 0.0075

so mass = 0.0075 x 90.8 = 0.681 g