Larutan NH4​OH dengan konsentrasi 0,2 M memilki pH sebesar 11+log 2. Jika 2​=1,4 dan 5​=2,2maka tentukanlah: a. Nilai Kb​ larutan tersebut b. Persentase banyaknya  yang terionisasi (derajad ionisasinya)

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Larutan begin mathsize 14px style N H subscript 4 O H end style dengan konsentrasi 0,2 M memilki pH sebesar 11+log 2. Jika begin mathsize 14px style square root of 2 equals 1 comma 4 end style dan begin mathsize 14px style square root of 5 equals 2 comma 2 end stylemaka tentukanlah:

a. Nilai begin mathsize 14px style K subscript b end style larutan tersebut
b. Persentase banyaknya begin mathsize 14px style N H subscript 4 O H end style yang terionisasi (derajad ionisasinya)undefined 

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Pembahasan

Larutan begin mathsize 14px style N H subscript 4 O H end style merupakan larutan basa lemah. Konsentrasi begin mathsize 14px style O H to the power of minus sign end style pada basa lemah, dipengaruhi oleh konstanta disosiasi basa (Kb). Untuk menentukan Kb, dapat menggunakan rumus hubungan antara Kb dan  begin mathsize 14px style O H to the power of minus sign end style. Namun sebelumnya, perlu dihitung konsentrasi begin mathsize 14px style O H to the power of minus sign end style dalam begin mathsize 14px style N H subscript 4 O H end style.

Penentuan konsentrasi begin mathsize 14px style O H to the power of minus sign end style dalam begin mathsize 14px style N H subscript 4 O H end style


begin mathsize 14px style table attributes columnalign right center left columnspacing 0px end attributes row cell pH and pOH end cell equals 14 row pOH equals cell 14 minus sign pH end cell row blank equals cell 14 minus sign left parenthesis 11 plus log 2 right parenthesis end cell row blank equals cell 3 minus sign log 2 end cell row blank blank blank row pOH equals cell negative sign log open square brackets O H to the power of minus sign close square brackets end cell row cell 3 minus sign log 2 end cell equals cell negative sign log open square brackets O H to the power of minus sign close square brackets end cell row cell open square brackets O H to the power of minus sign close square brackets end cell equals cell 2 cross times 10 to the power of negative sign 3 end exponent end cell end table end style 
 

a. Penentuan nilai Kb


begin mathsize 14px style table attributes columnalign right center left columnspacing 0px end attributes row cell open square brackets O H to the power of minus sign close square brackets end cell equals cell square root of K subscript b cross times M subscript N H subscript 4 O H end subscript end root end cell row cell K subscript b end cell equals cell open square brackets O H to the power of minus sign close square brackets squared over M subscript blank subscript N H subscript 4 O H end subscript end subscript end cell row blank equals cell fraction numerator left parenthesis 2 cross times 10 to the power of negative sign 3 end exponent right parenthesis squared over denominator 0 comma 2 space mol forward slash L end fraction end cell row blank equals cell fraction numerator 4 cross times 10 to the power of negative sign 6 end exponent over denominator 2 cross times 10 to the power of negative sign 1 end exponent end fraction end cell row blank equals cell 2 cross times 10 to the power of negative sign 5 end exponent end cell end table end style 
 

Jadi, nilai Kb begin mathsize 14px style N H subscript 4 O H end style sebesar begin mathsize 14px style 2 cross times 10 to the power of negative sign 5 end exponent end style

b. Derajad ionisasi begin mathsize 14px style N H subscript 4 O H end style (alpha


begin mathsize 14px style table attributes columnalign right center left columnspacing 0px end attributes row alpha equals cell square root of K subscript b over M subscript N H subscript 4 O H end subscript end root end cell row blank equals cell square root of fraction numerator 2 cross times 10 to the power of negative sign 5 end exponent over denominator 0 comma 2 end fraction end root end cell row blank equals cell square root of fraction numerator 2 cross times 10 to the power of negative sign 5 end exponent over denominator 2 cross times 10 to the power of negative sign 1 end exponent end fraction end root end cell row blank equals cell square root of 1 cross times 10 to the power of negative sign 4 end exponent end root end cell row blank equals cell 1 cross times 10 to the power of negative sign 2 end exponent end cell row blank equals cell 0 comma 01 end cell end table end style 


Dari perhitungan di atas, diperoleh nilai derajad ioniasi (alpha) dari begin mathsize 14px style N H subscript 4 O H end style sebesar 0,01 atau 1%.space 

jadi, Nilai Kb begin mathsize 14px style N H subscript 4 O H end style adalah sebesar 2 cross times 10 to the power of negative sign 5 end exponent, dan persentase banyaknya begin mathsize 14px style N H subscript 4 O H end style yang terionisasi (derajad ionisasinya) adalah sebesar 0,01 atau 1%.

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Tentukan harga konsentrasi ion OH− yang terdapat dalam larutan dimetil amino (CH3​)2​NH 0,01 M jika harga Kb​ larutan tersebut = 5,1×10−4! Tentukan pula harga derajat ionisasi dari larutan tersebut!

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