Calculate the pH of the solution resulting when 10.00 ml of 0.150 M NaOH (aq) is added to 25.00 ml of 0.100 M?

Question

HCOOH (aq).Use KA 1.8X10

Answer 1

its a buffer or a mixture of acid and its salt

HCOOH +NaOH ---> Na COOH

so 10 ml of 0.150M contain 0.0015moles of NaOH

This neutralizes 0.0015moles of HCOOH
You have so 0.0015moles of salt
the acid remaining is 0.025*0.1 -0.0015 = 0.001moles of acid

you have the formula for pH

pH = pK +log [salt]/[acid]

here pH = pK +log 0.0015/0.001 = pk +log 1.5 =pK +0.18

I have found but you did not give pK =3.75
so pH =3.75+0.18= 3.93

Look at the pK