It is found that 24.68 mL of 0.1165M NaOH is needed to titrate 0.2931 g of an unknown acid to the phenolphthalein end point. Calculate the equivalent mass of the acid.

101.8 g/ equiv

Explanation:

The equivalence point for a titration is reached when

# Equivalents of  Acid = # Equivalents of  Base

and the number of equivalents is defined as the mass divided into the equivalent weight.

The equivalent weight is the molecular weight divided into the number of protons ( H⁺) per formula of the acid and number of hydroxyl groups ( OH⁻ ) for the base.

Now we are not told in this question the number of protons, H⁺, so lets assume our unknown is monoprotic acid.

From the formulas above we can set the following expression

#~Equiv Acid = mass/ equiv Weight = # Equiv NaOH = V NaOH x Equiv Wt

Since we used 24.68 mL of .1165 M of NaOH for the titration the equivalents are:

= ( 24.68 mL x  1 L /1000 mL ) x 0.1165 mol / L NaOH  

= 0.0029 mol = 0.00288 equiv ( 1 OH⁻ )

and,

Equiv acid = 0.0029 equiv

Equiv acid  = 0.2931 / Equiv Weight

Equiv Weight = 0.2931  q / 0.00288 equiv

⇒ Equiv Weight = 101.8 g/ equiv