# Weak base/ Strong acid pH

Question: TRIS is a weak base. Compute pH of a solution containing .050 moles of TRIS and .020 moles of HCl in a total volume of 2L. pKb for TRIS is 5.92. Use RNH2 to represent TRIS.

Equation: pH = pKa+ log ([conjugate base]/[acid])

Work:
change equation for pOH:
pH = 14-pOH
pOH = pKb + log ([conjugate acid]/[base])
pOH = 5.02 + log (.010/.025) <---- I converted to mol/L for these values
pOH = 5.52
pH = 8.48

Can someone show me where I went wrong? Thanks!

Borek
Mentor
What are your conjugated acid and base? What is your base concentration? (Hint: write reaction equation).

Borek

Last edited by a moderator:
They don't say what the conjugate acid and base is, except that the base is RNH2, so then its conjugate acid would be RNH3, right?

The base concentration would be .05/2 = .025 M.

The reaction equation would be
RNH2 + HCl <--> H2O + RNH3
Initial:
.25M .1M 0
Change:
-.1M -.1M .1M
Final:
.15M 0M .1M

pOH = pKb + log (.1/.15)
pOH = 5.92 + log (.1/.15)
pOH = 5.74
pH = 8.26

...

That's it! Thank you! :D :D

Why is it better to use a weak base(such as calcium carbonate) to neutralise strong acid (such as hcl) rather than strong base?

Thank you so much

symbolipoint
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