[Fox_Hound]

Is my pH right, because i never did an example like this and could someone please shed some light on how to do this type of titration problem?

Fifty (50.00) milliliters of 0.400 M HCO2H is mixed with 50.00 mL of 0.200 M sodium formate (NaCHO2). Then 40.00 mL of 0.200 M NaOH is added to this solution. Calculate the pH of the final mixture.
My answer is pH=3.576

[Borek]

That's not the correct answer. Please show how you got it.

[Fox_Hound]

Sorry I didn't give the Ka(1.77E-4).The way I got it was by doing                                                                                       50.00ml HCO2H * .400M HCO2H =20mM HCO2H                                                                                       40.00ml NaOH * .200M NaOH =8mM OH                                                                                              20-8=12mM HCO2H left                                                                        pH=-log(1.77E-4) + log(8/12)    pH=3.576       

[Borek]

You are right about the amount of the acid present, but your amount of the conjugate base (salt) is off.

[Fox_Hound]

where do i go from there considering the amount of salt

[AWK]

NaOH reacts with HCOOH and HCOONa is formed

[Borek]

Other than miscalculating number of mmoles of salt you are OK.

[Fox_Hound]

How would I know what amount of mMoles of salt I would use.Would the correct answer be pH=4.706.Can someone please just show me how to set up and solve the problem.

[AWK]

Just calculate stoichiometry of neutralization and add HCOONa together

[Fox_Hound]

so when i do it i just subtract the total number of mMoles of salt and solve from there.What is the right way to calculate it then.Because I keep on coming up with a pH of 4.706.

[Borek]

Show how you are getting 4.706.

[Fox_Hound]

my new answer is pH=3.313.Igot it by adding the mM of HCO2H and NaCO2H together then subtracted the mM of NaOH then plugging in the values. -log(1.77x10^-4)+log(8/22) =3.313

[AWK]

10 + 8 = 22 ? good result!

[Fox_Hound]

if i'm doing it wrong then whats the answer

[Borek]

You are juggling numbers, it never helps.

You have to calculate number of mmoles of acid in the final solution, and number of mmoles of conjugate base in the final solution, to plug them into Henderson-Hasselbalch equation.

How many mmoles of acid were present initially? How many were neutralized? How many were left?

How many mmoles of the salt were present initially? How many were produced during neutralization? How many in total?

And please - don't cut corners. Answer every of these questions separately and list your answers. Otherwise we can't show you what and where you are doing wrong.