Weak base/ Strong acid pH

[enigmatic]

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. pK_b 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

Real Answer: 8.26

Can someone show me where I went wrong? Thanks!

 

[Borek]

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

Borek

 

[enigmatic]

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

 

[Booo]

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]

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

Generally, a weak base is not better and not worse. The choice depends on the application. Why do you ask?

If you use calcium carbonate to neutralize a strong acid, you hopefully are not worried about the presence of calcium, and the resulting carbon dioxide may be mostly insoluble, but beware of bubbling or foaming. Depending on quantities involved of the base(calcium carbonate) and the acid, you might be choosing to produce a buffer; is that what you wanted? Did you want some bicarbonate present?