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Dalam laboratorium kimia, larutan penyangga bisa dibuat dengan cara mencampurkan asam lemah dengan garamnya atau basa lemah dengan garamnya. Seorang laboran kimia ingin membuat larutan penyangga dengan pH 5 - log 2. Dia mencampurkan sebanyak 0,49 gram garam begin mathsize 14px style C H subscript 3 COOX end style ke dalam 200 mL larutan begin mathsize 14px style C H subscript 3 C O O H end style 0,025 M begin mathsize 14px style left parenthesis K subscript a space C H subscript 3 C O O H equals 2 cross times 10 to the power of negative sign 5 end exponent right parenthesis end style. Unsur X yang terkandung dalam garam tersebut adalah ... (begin mathsize 14px style italic A subscript r end style: C = 12, H = 1, O = 16, Mg = 24, Ca = 40, K = 39, Na = 23, Ba = 137)

I. Solichah

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Jawaban terverifikasi

Jawaban

 unsur X yang terkandung dalam garam tersebut adalah K.

Pembahasan

Menentukan nilai molaritas begin mathsize 14px style H to the power of plus sign end style:

begin mathsize 14px style table attributes columnalign right center left columnspacing 0px end attributes row cell italic p H end cell equals cell negative sign log space open square brackets H to the power of plus sign close square brackets end cell row cell 5 minus sign log space 2 end cell equals cell negative sign log space open square brackets H to the power of plus sign close square brackets end cell row cell negative sign log space 2 cross times 10 to the power of negative sign 5 end exponent end cell equals cell negative sign log space open square brackets H to the power of plus sign close square brackets end cell row cell open square brackets H to the power of plus sign close square brackets end cell equals cell 2 cross times 10 to the power of negative sign 5 end exponent end cell end table end style 

Menentukan nilai mol larutan:

begin mathsize 14px style table attributes columnalign right center left columnspacing 0px end attributes row cell n subscript C H subscript 3 C O O H end subscript end cell equals cell M cross times V end cell row blank equals cell 0 comma 025 cross times 200 end cell row blank equals cell 5 space mmol end cell row blank equals cell 5 cross times 10 to the power of negative sign 3 end exponent space mol end cell end table end style 

begin mathsize 14px style table attributes columnalign right center left columnspacing 0px end attributes row cell open square brackets H to the power of plus sign close square brackets end cell equals cell K subscript a cross times n subscript C H subscript 3 C O O H end subscript over n subscript C H subscript 3 C O O to the power of minus sign end subscript end cell row cell 2 cross times 10 to the power of negative sign 5 end exponent end cell equals cell 2 cross times 10 to the power of negative sign 5 end exponent cross times fraction numerator 5 cross times 10 to the power of negative sign 3 end exponent over denominator n subscript C H subscript 3 C O O to the power of minus sign end subscript end fraction end cell row cell n subscript C H subscript 3 C O O to the power of minus sign end subscript end cell equals cell fraction numerator 10 cross times 10 to the power of negative sign 8 end exponent over denominator 2 cross times 10 to the power of negative sign 5 end exponent end fraction end cell row blank equals cell 5 cross times 10 to the power of negative sign 3 end exponent end cell end table end style 

  

Menentukan nilai begin mathsize 14px style italic A subscript r end style X:

begin mathsize 14px style table attributes columnalign right center left columnspacing 0px end attributes row cell n subscript C H subscript 3 COOX end subscript end cell equals cell m over italic M subscript r end cell row cell 5 cross times 10 to the power of negative sign 3 end exponent end cell equals cell fraction numerator 0 comma 49 over denominator italic M subscript r end fraction end cell row cell italic M subscript r end cell equals cell 98 space g space mol to the power of negative sign 1 end exponent end cell end table end style 

begin mathsize 14px style table attributes columnalign right center left columnspacing 0px end attributes row cell italic M subscript r space C H subscript 3 COOX end cell equals cell left parenthesis 2 cross times italic A subscript r space C right parenthesis plus left parenthesis 3 cross times italic A subscript r space H right parenthesis plus left parenthesis 2 cross times italic A subscript r space O right parenthesis plus left parenthesis 1 cross times italic A subscript r space X right parenthesis end cell row 98 equals cell left parenthesis 2 cross times 12 right parenthesis plus left parenthesis 3 cross times 1 right parenthesis plus left parenthesis 2 cross times 16 right parenthesis plus left parenthesis 1 cross times italic A subscript r space X right parenthesis end cell row 98 equals cell 24 plus 3 plus 32 plus italic A subscript r space X end cell row 98 equals cell 59 plus italic A subscript r space X end cell row cell italic A subscript r space X end cell equals 39 end table end style 

Sehingga unsur X adalah unsur K dengan nilai begin mathsize 14px style italic A subscript r end style 39.

Jadi, unsur X yang terkandung dalam garam tersebut adalah K.

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Jika 400 mL CH3​COOH 0,1 M direaksikan dengan 100 mL Ca(OH)2​ 0,1 M dengan Ka​=2x10−5, tentukan pH larutan yang terjadi.

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