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Equilibrium

Question
CBSEENCH11006566
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At 298 K, calculate the pH of 0.200 M solution of methylamine, CH3NH2(ionisation constant = 4·4×105).

Solution
In aqueous solution.
CH subscript 3 NH subscript 2 space plus space straight H subscript 2 straight O space space rightwards harpoon over leftwards harpoon space space space CH subscript 3 NH subscript 3 superscript plus space plus space OH to the power of minus
If C is the initial concentration and a is the degree of dissociation, then,
space space space space space space space space space space space space space space space CH subscript 3 NH subscript 2 space space space space plus space space straight H subscript 2 straight O space space space space rightwards harpoon over leftwards harpoon space space space space space space CH subscript 3 NH subscript 3 superscript plus space space space space plus space space space space space OH to the power of minus Initial space conc space space space space space space space straight C space mol space straight L to the power of negative 1 end exponent space space space space space space space space space space space space space space space space space space space space space space space space 0 space space space space space space space space space space space space space space space space space space 0 space space space space space space space space Equilibrium space space space space space space space space straight C minus Cα space space space space space space space space space space space space space space space space space space space space space space space space space space space Cα space space space space space space space space space space space space space space space space Cα
straight K subscript straight b space equals space fraction numerator open square brackets CH subscript 3 NH subscript 3 superscript plus close square brackets space open square brackets OH to the power of minus close square brackets over denominator open square brackets CH subscript 3 NH subscript 2 close square brackets end fraction space space space space space equals space fraction numerator Cα space cross times space Cα over denominator straight C space minus space Cα end fraction space equals space fraction numerator straight C squared straight alpha squared over denominator straight C left parenthesis 1 minus straight alpha right parenthesis end fraction space equals space fraction numerator Cα squared over denominator 1 minus straight alpha end fraction or space space space straight K subscript straight b space end subscript space equals space Cα squared space space space space space space space space space space space space space open square brackets because space 1 minus straight alpha space equals space 1 close square brackets or space space space space straight alpha space equals space square root of straight K subscript straight b over straight C end root space space space space space space space space space space space equals square root of fraction numerator 4.4 space cross times 10 to the power of negative 5 end exponent over denominator 0.200 end fraction end root space space space space space space space space space space space equals 1.48 space cross times 10 to the power of negative 2 end exponent

therefore space space open square brackets OH to the power of minus close square brackets space equals space straight C space cross times space straight alpha space space space space space space space space space space space space space space space equals 0.200 space cross times space 1.48 space cross times space 10 to the power of negative 2 end exponent space space space space space space space space space space space space space space space equals 2.96 space cross times 10 to the power of negative 3 end exponent straight M We space know comma space space space space open square brackets straight H subscript 3 straight O to the power of plus close square brackets space open square brackets OH to the power of minus close square brackets space equals space straight K subscript straight w or space space space open square brackets straight H subscript 3 straight O to the power of plus close square brackets space equals space fraction numerator straight K subscript straight w over denominator open square brackets OH to the power of minus close square brackets end fraction space equals space fraction numerator 1 space cross times space 10 to the power of negative 14 end exponent over denominator 2.96 space cross times 10 to the power of negative 3 end exponent end fraction space space space space space space space space space space space space space space space space space equals space 3.378 space cross times space 10 to the power of negative 12 end exponent space space space space space space space space space space space space space space space space space space space
Also comma space space space space pH space equals negative log space open square brackets straight H subscript 3 straight O to the power of plus close square brackets space space space space space space space space equals negative log space left parenthesis 3.378 space cross times space 10 to the power of negative 12 end exponent right parenthesis space space space space space space space space equals space minus log space left square bracket 3.378 space plus space log space 10 to the power of negative 12 end exponent right square bracket space space space space space space space space equals negative open square brackets log space 3.378 space minus space 12 space log space 10 close square brackets space space space space space space space space equals negative left square bracket 0.5287 space minus space 12 right square bracket space space space space space space space space equals space 11.4712 space asymptotically equal to space 11.47

Some More Questions From Equilibrium Chapter

Which measurable property becomes constant in water vapour equilibrium?

Give an example from daily life in which there is gas solution equilibrium?

A crystal of common salt of a given mass is kept in its aqueous solution. After 24 hours, its mass remains the same. Is the crystal in equilibrium with the solution?

What is the role of forward and backward reactions at equilibrium of a reversible reaction?

What is a forward reaction?

What condition favours a reversible reaction?

What is a backward reaction?

What type of equilibrium is attained in a reversibe chemical change?

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