[xcxo]

Hi,

I just have a few questions concerning 2 homework problems I'm having trouble with, and I'd be really grateful if anyone could explain them to me or give me a nudge in the right direction.

THERMOCHEMISTRY
a system comprised of 2.578 moles of Argon (g) cools from 195°C to 50°C at a constant pressure of 2.0 atm. The molar heat capacity for Argon is 20.80 J K^-1 mol ^-1.

calculate w
w = PΔV = nRT
= (2.578)(8.314)(-145)
= -3107

I'm having trouble with sign convention - the number I get is correct but I'm wondering why the sign convention should be positive, and not negative like I've obtained?

BUFFER SOLUTION
calculate the pH of a solution made by mixing 130.00 mL of 1.100 M HCOOH with 65.00 mL of 0.900M NaOH.

My attempt wasn't the right answer, but I'm going to post it in hopes that someone can spot where I made the error.
C HA = 0.130 L x 1.100 M = 0.143 M
C A = 0.0650 L x 0.900 M = 0.0585 M
Ka of HCOOH = 1.8 E-4
pKa = -log(Ka) = 3.744

pH = pKa + log ([A]/[HA])
= 3.744 + log (0.0585/0.143)
= 3.36

The correct answer should be 3.59. Can anyone give me a hint to where I made my mistake?

[Borek]

where I made the error

What is the acid in the solution? What is the conjugate base? Where does the base come from?

[xcxo]

I think the (weak) acid is HCOOH, and the conjugate base would be NaOH. So the base would come from NaOH then?

[Borek]

I think the (weak) acid is HCOOH, and the conjugate base would be NaOH.

Nope.

What is the definition of the conjugate base?

http://www.chembuddy.com/?left=pH-calculation&right=bronsted-lowry-theory

[xcxo]

So HCOO^- is the conjugate base of the weak acid HCOOH?

HCOO^- + H₂O  HCOOH + OH-

[Borek]

Yes. And where does the HCOO^- come from?

[xcxo]

It would come from the dissociation of HCOOH, so
HCOOH  HCOO^- + H⁺

[Borek]

So if it is present, is it possible for the original concentration of HCOOH to remain unchanged?