Posted by **harry** on Sunday, April 26, 2009 at 5:40am.

Calculate the equilibrium constant for the weak base CH3NH2, if a solution of the base with an initial concentration of 7.05×10-4 M has a [CH3NH3+] of 0.000379 M (make an exact calculation assuming that initial concentration is not equal to the equilibrium concentration).

CH3NH2 + H2O = CH3NH3+ + OH-

i don't know how to do this problem help is appreciated

- CHEM -
**DrBob222**, Sunday, April 26, 2009 at 5:57pm
I'm not exactly sure about this problem because I don't understand the portion in the parentheses. My assumption is that the system has not reached equilibrium (after-all that's what the problem states) so the concns are as listed.

So I would write the Kb expression.

Kb = (CH3NH3^+)(OH^^-)/(CH3NH2)

and plug in the values given.

If CH3NH3^+ is 0.000379, that must be the concn of OH^-, too, and CH3NH2 must be what we started with minus the amount of CH3NH3^+ formed or 0.000705-0.000379. I worked through the problem and obtained 4.41 x 10^-4 for Kb. My OLD quant book (copyright 1992) gives a value of 4.4 x 10^-4. Fairly close. Check my thinking.

## Answer This Question

## Related Questions

- college chemistry - Calculate the [CH3NH3+] of a 7.02×10-3 M solution of the ...
- Chem - Estimate the equilibrium constant for the weak base (CH3)2NH, if a 1.59×...
- chemistry - Calculate the equilibrium constant for the weak base C5H5N, if a ...
- Chemistry - Estimate the equilibrium constant for the weak acid HCN, if a 4.68×...
- AP Chemistry - CH3NH2(aq)+H2O(l)=>CH3NH3+(aq)+OH-(aq) Kb=4.4 x 10^-4 ...
- chemistry - CH3NH2(aq)+H2O(l)=>CH3NH3+(aq)+OH-(aq) Kb=4.4 x 10^-4 Methylamine...
- chemistry - Estimate the equilibrium constant for the weak acid HCO2H, if a 2.19...
- chemistry - Calculate the pH of a 5.75×10-2 M solution of the weak acid HCN (...
- chemistry - Calculate the [OCl-] of a 7.03×10-2 M solution of the weak acid ...
- Chem - Methylaime, CH3NH2. If a 0.100 mol/L solution of methyamine has a pH of ...

More Related Questions