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Sebanyak 1,92 gram  dicampurkan ke dalam 100 mL la...

Sebanyak 1,92 gram begin mathsize 14px style C subscript 2 H subscript 5 C O O italic X end style dicampurkan ke dalam 100 mL larutan begin mathsize 14px style C subscript 2 H subscript 5 C O O H end style 2 M (begin mathsize 14px style K subscript a equals 1 cross times 10 to the power of negative sign 5 end exponent end style) sehingga membentuk larutan penyangga dengan pH = 4. Unsur X yang terkandung dalam garam tersebut adalah ....

(begin mathsize 14px style A subscript r colon space C equals 12 space g space mol to the power of negative sign 1 end exponent comma end style begin mathsize 14px style H equals 1 space g space mol to the power of negative sign 1 end exponent comma end style begin mathsize 14px style O equals 16 space g space mol to the power of negative sign 1 end exponent comma end style begin mathsize 14px style K equals 39 space g space mol to the power of negative sign 1 end exponent comma end style begin mathsize 14px style Li equals 7 space g space mol to the power of negative sign 1 end exponent comma end style begin mathsize 14px style Ba equals 137 space g space mol to the power of negative sign 1 end exponent comma end style begin mathsize 14px style Ca equals 40 space g space mol to the power of negative sign 1 end exponent comma end style dan begin mathsize 14px style Na equals 23 space g space mol to the power of negative sign 1 end exponent end style)

  1. begin mathsize 14px style K end style 

  2. begin mathsize 14px style Li end style 

  3. begin mathsize 14px style Ba end style 

  4. begin mathsize 14px style Ca end style 

  5. begin mathsize 14px style Na end style 

Jawaban:

Larutan penyangga asam memiliki komponen asam lemah dan basa konjugasinya (biasanya berbentuk garam). Pada soal asam lemahnya adalah begin mathsize 14px style C subscript 2 H subscript 5 C O O H end style sedangkan garamnya begin mathsize 14px style C subscript 2 H subscript 5 C O O italic X end style. pH larutan ini diketahui = 4, maka

begin mathsize 14px style table attributes columnalign right center left columnspacing 0px end attributes row pH equals cell negative sign log space open square brackets H to the power of plus sign close square brackets end cell row 4 equals cell negative sign log space open square brackets H to the power of plus sign close square brackets end cell row cell left square bracket H plus right square bracket end cell equals cell 10 to the power of negative sign 4 end exponent end cell end table end style 

Pada larutan penyangga berlaku:

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 fraction numerator left square bracket asam space lemah right square bracket over denominator left square bracket basa space konjugasi right square bracket end fraction end cell row cell 10 to the power of negative sign 4 end exponent end cell equals cell 10 to the power of negative sign 5 end exponent cross times fraction numerator left parenthesis 2 cross times 0 comma 1 right parenthesis forward slash 100 over denominator mol space C subscript 2 H subscript 5 COOX forward slash 100 end fraction end cell row 10 equals cell fraction numerator 0 comma 2 over denominator mol space C subscript 2 H subscript 5 COOX end fraction end cell row cell mol space C subscript 2 H subscript 5 COOX end cell equals cell 0 comma 02 space mol end cell end table end style 

Selanjutnya kita cari massa molekul relatif garam tersebut,

begin mathsize 14px style table attributes columnalign right center left columnspacing 0px end attributes row mol equals cell massa over M subscript r end cell row cell M subscript r end cell equals cell fraction numerator 1 comma 92 space g over denominator 0 comma 02 space mol end fraction end cell row cell M subscript r end cell equals 96 end table end style 

begin mathsize 14px style table attributes columnalign right center left columnspacing 0px end attributes row cell M subscript r space C subscript 2 H subscript 5 COOX end cell equals cell 3 A subscript r space C and 5 A subscript r space H and 2 A subscript r space O and A subscript r space X end cell row 96 equals cell 3 left parenthesis 12 right parenthesis plus 5 left parenthesis 1 right parenthesis plus 2 left parenthesis 16 right parenthesis plus A subscript r space X end cell row cell A subscript r space X end cell equals cell 96 minus sign 73 end cell row cell A subscript r space X end cell equals 23 end table end style 

Unsur yang memiliki massa atom relatif 23 adalah Natrium.

Jadi, jawaban yang benar adalah E.

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