Answer:

0.127 M.

Explanation:

The balanced equation for the reaction is given below:

H3PO4 + 3KOH —› K3PO4 + 3H2O

From the balanced equation above, we obtained the following data:

Mole ratio of acid, H3PO4 (nA) = 1

Mole ratio of base, KOH (nB) = 3

Data obtained from the question include:

Volume of acid, H3PO4 (Va) = 78.5 mL

Molarity of acid, H3PO4 (Ma) =...?

Volume of base, KOH (Vb) = 134 mL

Molarity of base, KOH (Mb) = 0.224 M

The concentration of the acid, H3PO4 can be obtained as follow:

MaVa / MbVb = nA/nB

Ma x 78.5 / 0.224 x 134 = 1/3

Cross multiply

Ma x 78.5 x 3 = 0.224 x 134 x 1

Divide both side by 78.5 x 3

Ma = (0.224 x 134 x 1) /(78.5 x 3)

Ma = 0.127 M

Therefore, the concentration of the acid, H3PO4 is 0.127 M.