Posts filed under 'Percent Composition'
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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%
October 13th, 2008
Stoichiometry calculation of acetyl salicylic acid is easier we solve if we know the complete reaction that involve. The synthesis of acetyl salicylic acid (ASA) is categorized as an esterification of salicylic acid and acetic anhydride that yielding acetyl salicylic acid and acetic acid as by product, a few drops of sulfuric acid often added as a catalyst. This technique is usually employed in undergraduate teaching labs.
Now examine this problem on how you can calculate the theoretical yield of acetylsalicylic synthesis.
“Chemistry student reacts 4.81 g of salicylic acid with an excess of acetic anhydride in the presence of a few drops of sulfuric acid to compose acetylsalicylic acid (aspirin), which the student then purifies. (1) Calculate the theoretical yield mass of product (2) if the student only get aspirin 5,781 g calculate the %yield of the student result ”
Free Chemistry Tutor
In the synthesis of aspirin we usually used excess acetic anhydride; this should be done to ensure that the salicylic acid will be completely reacted and we get the maximum result. But unfortunately there are not chemical reactions that 100% complete, many factors influence them. As the result, we have the theoretical result and % yield of the reaction. Let’s we solve the problem.
The balanced reaction :
+ (CH3CO)2O -> 
+ CH3COOH
Firstly calculate the molecular mass of the salicylic acid and the acetylsalicylic acid (aspirin)
Mr salicylic acid (C7H6O3)
= (7 x 12.01) + (6 x 1.008) + (3 x 16.00)
= 138.118 g/mol
Mr acetylsalicylic acid (C9H8O4)
= (9 x 12.01) + (8 x 1.008) + (4 x 16.00)
= 180.154 g/mol
Secondly calculate the moles of salicylic acid
Moles salicylic acid (C7H6O3)
= mass / molar mass
= 4.81 / 138.118
= 0.035 mol
the mole ratio of salicylic acid and acetylsalicylic acid (aspirin) is 1:1 thus from this stoichiometry we can calculate the moles of acetylsalicylic acid (aspirin) that produced.
Moles of acetylsalicylic acid (aspirin)
= 1/1 x moles of salicylic acid (C7H6O3)
= 1/1 x 0.035 mol
= 0.035 mol
The theoretical mass of acetic salicylic acid (aspirin) obtains by multiplying the moles of aspirin with its molecular mass.
Theoretical mass of acetylsalicylic acid (C9H8O4)
= moles x molar mass
= 0.035 mol x 180.154 g/mol
= 6.274 g
The mass of percentage yield calculated by dividing actual mass of the aspirin that the student got with the theoretical that we solve above in the percent, and we have
The % yield of the aspirin
= actual mass / theoretical yield x 100%
= 5,781 / 6.274 x 100%
= 92.143 %
Stoichiometry calculation of theoretical yield and mass percentage yield of acetylsalicylic synthesis from the calculation above we have 6.274 g and 92.143 % respectively.
See also :
Calculate the actual mass of aspirin
June 20th, 2008
Molarity of concentrated of sulfuric acid (H2SO4) can be calculate if the density of the solution and its percentage concentration are known. We usualy calculate the molarity of concentrated sulfuric acid when we will preapare the more dilute of H2SO4 solution. We do this more in the analytical experiments. But the easy way is to take a look on the label of the container of the concentrated sulfuric acid solution. From this we can get the density and the percent by mass of this solution. How we can calculate the molarity of concentrated H2SO4? just take the example below:
“Calculate the molarity of concentrated of H2SO4 (sulfuric acid solution) that has the spesific gravity 1.84?”
Here the answer:
First we should now the definition of specific gravity (SG). SG is defined as the ratio of density of the material to the density of water at a specified temperature 4oC. Specific Gravity is dimensionless and can be expressed as
SG = p / pH2O
where
p = density of the material
p = density of H2O at 4oC= 1000 Kg/m3 = 1 g/cm3
from the formula above we can calculate the density of the concentrated of sulfuric acid solution as
density of concentrated H2SO4 solution
= SG x pH2O
= 1.84 x 1 g/cm3
= 1.84 g/cm3
= 1.84 g/mL
Assume that we have 100 mL of the concentrated of sulfuric acid thus we can calculate the mass of this solution
mass of solution
= volume x density
= 100 mL x 1.84 g/mL
= 184 g
mass of H2SO4 in this solution can be calculate by multipliying the mass of the solution with its concentration
mass H2SO4
= mass of the solution x percentage
= 184 g x 98%
= 180.32 g
and the moles of H2SO4 are
= mass / molar mass
= 180.32 g / 98.086
= 1.838 mol
and finaly the concentration of concentrated H2SO4 solution can be calculate by dividing the moles with the volume of the solution
Molarity H2SO4 concentrated = 1.838 mol / 0.1 L
Molarity H2SO4 concentrated = 18.38 M
June 12th, 2008
Chemistry Tutor Site Problem
Methyl salicylate(oil of wintergreen) is prepared by heating salicylic acid(C7H6O3) with methanol (CH3OH)? In an experiment, 6.252g of salicylic acid is reacted with an excess of methanol. How many grams of oil of wintergreen is produced if the percent yield of the reaction is 68.63%?
Chemistry Tutor Site Help
When you solve the problem using stoichiometry calculation, it means that you calculate the theoretical yield. The percent yield of the reaction gives you the actual yield of the product. To find the mass of the methyl salicylate we need first to calculate the molar mass of each species are:
Mr C7H6O3 = 138.123 g/mol
Mr C8H8O3 = 152.1494 g/mol
Mr CH3OH = 32.04 g/mol
We have 6.252g of salicylic acid to be reacted with excess of methanol thus the moles of salicylic acid are
moles salicylic acid = 6.252 g / 138.123 g/mol = 0.045 mol
the balanced reaction is :
C7H6O3 + CH3OH –> C8H8O3 + H2O
because the mole ration between C7H6O3 and C8H8O3 is 1:1 thus 0.045 mol of C7H6O3 will theoretically produce 0.045 mol of C8H8O3 too, and the mass of C8H8O3:
mass C8H8O3 = 0.045 mol x 152.1494 g/mol = 6.887 grams
remember that this is the theoretically mass of C8H8O3, because the yield of the reaction is 68.63% so the actual mass of C8H8O3 tha has been produced is:
actual mass C8H8O3 = 68.63% x 6.887 grams = 4.726 gr
see also
Stoichiometry Calculation of theoretical and percent yield of aspirin
May 28th, 2008
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