Mass Percentage Question? How To Calculate The Mass Percentage in Mixture of KBr-KI
October 13th, 2008
Chemistry Tutor Site-Chemistry Help
A Student has a sample that contains both KBr and KI in unknown quantities. If the Student takes the sample with a total mass of 5.00g and contains 1.51g K, what are the percentages of KBr and KI in the sample by mass?
Chemistry Tutor Site-Answer
To solve this problem we use the mass of K (Potassium) as a reference, because both KBr and KI contain K.
Mr KBr = 39.10 + 79.90 = 119 g/mol
Mr KI = 39.10 + 126.9 = 166 g/mol
We assume that the mass of K Br are x g, thus the mas of KI are (5.00-x) g and you should remember how you calculate the mass of elements in the compound using this formula:
mass of K in KBr = Ar K/Mr KBr x Mass of KBr
Using the same method we can calculate the mass of K inKI, thus the total mass of K in KBr and KI are:
Total mass of K = mass of K in KBr + mass of K in KI
1.51 g = (ArK/MrKBr).mass of KBr + (ArL/MrKI).mass of KI
1.51 g = (39.10/119).x + (39.10/166).(5.00-x) g
1.51 g = 0.329x + 0.236 (5.00-x) g
1.51 g = 0.329x + 1.18 g - 0.236x
0.33 g = 0.093x
x = 3.548 g
Because x = mass of KBr thus the mass of KBr = 3.548 g
mass of KI = (5.00-x) = 5.00 - 3.548 = 1.452 g
and the mass percentage of KBr and KI in the sample are:
%KBr = mass KBr/mass sample .100% = 3.548/5.00 . 100% = 70.96%
%KI = 100% - 70.96% = 29.04%
Entry Filed under: Percent Composition
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