weak acid 4 g pH 2.56 250.0 mL pH 4.26 mol mass

weak acid 4 g pH 2.56 250.0 mL pH 4.26 mol mass

 

 

일양성자성 약산 4 g을 물에 녹여 pH 2.56이고,

전체 부피 250.0 mL인 용액을 만들었다.

 

 

Four grams of a monoprotic weak acid are dissolved in water

to make 250.0 mL of solution with a pH of 2.56.

The solution is divided into two equal parts, A and B.

Solution A is titrated with strong base to its equivalence point.

Solution B is added to solution A after solution A is neutralized.

The pH of the resulting solution is 4.26.

What is the molar mass of the acid?

 

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① 약산 용액의 초기농도(C)

② 약산의 Ka

③ 약산 용액의 pH (= 평형에서 [H^+] = x)

---> 이들 3개 data 중에서,

2개 data가 주어지면,

 

Ka = x^2 / (C–x) 관계식으로부터,

( 참고 https://ywpop.tistory.com/4294 )

 

나머지 1개 data의 값을 계산해낼 수 있다.

 

 

 

 

The solution is divided into two equal parts, A and B.

Solution A is titrated with strong base to its equivalence point.

Solution B is added to solution A after solution A is neutralized.

---> 반-당량점에 대한 정보 제공.

 

 

 

반-당량점(half equivalence point)에서,

pH = pKa

( 참고 https://ywpop.tistory.com/10228 )

 

Ka = 10^(-4.26) = 5.4954×10^(-5)

 

 

 

pH = –log[H^+] = 2.56

[H^+] = 10^(-2.56) = 0.002754 M

---> 평형에서 [H^+]

 

 

 

Ka = x^2 / (C–x)

 

C = (x^2 / Ka) + x

= ( 0.002754^2 / (5.4954×10^(-5)) ) + 0.002754

= 0.1408 M

---> 약산의 초기농도

 

 

 

(0.1408 mol/L) (250.0/1000 L) = 0.03520 mol 약산

( 참고 https://ywpop.tistory.com/7787 )

 

 

 

약산의 몰질량을 계산하면,

4 g / 0.0352 mol

= 113.64 g/mol

( 참고 https://ywpop.tistory.com/7738 )

 

 

 

답: 113.64 g/mol

 

 

 

 

[키워드] 반-중화점 기준문서, 반-당량점 기준문서, 반중화점 기준문서, 반당량점 기준문서, 약산의 초기농도 기준문서