Question

In: Chemistry

Provide 3 different approaches [a. Using mass % values; b. using the formula as a conversion...

Provide 3 different approaches [a. Using mass % values; b. using the formula as a conversion factor (moles F and moles Ca) and c. law of definite proportions (grams F and grams Ca)] to solving the following: Calculate the number of grams of fluorine in a sample of calcium fluoride that contains 2.01 g of calcium. Answer: 1.91 g F

Solutions

Expert Solution

a)
calcium fluoride is CaF2

mass % of Ca = molar mass of Ca*100 / molar mass of CaF2
                         = 40*100/78
                         = 51.3 %

mass % of F = 100 - 51.3 = 48.7 %

use:
mass % of Ca = mass of Ca*100 / total mass
51.3 = 2.01 * 100 / total mass
total mass = 3.92 g

To find mass of F use:
mass % of F= mass of F*100 / total mass
48.7 = mass of F * 100 / 3.92
mass of F = 1.91 g
Answer: 1.91 g

b)
Mass of Ca = 2.01 g
Molar mass of Ca = 40 g/mol
number of moles of Ca = mass / molar mass
               = 2.01/40
               = 0.05025 mol

In CaF2, 1 mol of Ca needs 2 mol of F
So, 0.05 mol Ca will need 0.05025*2 = 0.105 mol of F

Molar mass of F = 19 g/mol
Mass of F = molar mass of F * number of moles
     = 19 * 0.105
     =1.91 g
Answer: 1.91 g

c)
Ca and F reacts in 1:2 proportion
proportion of Ca= 1/2 * moles of F
mass of Ca / molar mass of Ca = mass of F/molar mass of F
2.01/40 = mass of F / 19
mass of F = 1.91 g
Answer: 1.91 g


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