Chemistry Practice Problems Ka and Kb Practice Problems Solution: The pH of a 1.00 M solution of urea, a weak organi...

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Solution: The pH of a 1.00 M solution of urea, a weak organic base, is 7.050.Calculate the Ka of protonated urea.

Problem

The pH of a 1.00 M solution of urea, a weak organic base, is 7.050.

Calculate the Ka of protonated urea.

Solution

We’re being asked to calculate the Ka of protonated urea (The pH of 1.00 M urea, a weak organic base is 7.050). 


Urea ((NH2)2CO) is a weak base. 

Remember that weak bases partially dissociate in water and that bases accept H+ from the acid (water in this case). 


The dissociation of Urea is as follows:


(NH2)2CO (aq) + H2O(l)  (NH2)2COH+ (aq) + OH-(aq)



From this, we can construct an ICE table. 

Remember that liquids are ignored in the ICE table.


Kb expression for (NH2)2CO  is:


Kb = productsreactants= [(NH2)2COH+][OH-][(NH2)2CO]

Liquids are ignored in the Kb expression.


Note that each concentration is raised by the stoichiometric coefficient: [(NH2)2CO], [OH] and [(NH2)2COH+] are raised to 1.


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