Question

In this experiment, 0.170 g of caffeine is dissolved in 10.0 mL of water. The caffeine is extracted from the aqueous solution three times with 5.0-mL portions of methylene chloride. Calculate the total amount of caffeine that can be extracted into the three portions of methylene chloride. Caffeine has a distribution coefficient of 4.6 between methylene chloride and water.

Answer 1

ans)

from above data that

Here the distributive coefficient denoted by k

given that the

K= 4.6 ml ,

volume of methylene chloride (v)= 5.0 ml ,

volume of water (V) = 10.0 ml ,

mass of caffeine in methylene chloride (m) , mass of caffeine in water (M)

we can written as

C1 = m/v (  concentration of caffeine in methylene chloride)    and

C2 = M/V ( concentration of caffeine in water )

substitute equation for the first extraction

first 0.1 70 g is equal to mass of caffeine in methylene chloride (m) + mass of caffeine in water (M1)

we take M1 = 0.170 - m1

K = C1 /C2 =   (m/v)    /    (M/V) =  

4.6 = (m / 5 ml)    /    (0.170 - m₁) / 10)   or m *10ml / (0.070 -M1 ) * 5 ml

0.170*4.6 - 4.6 m1 = 2 m1

0.782 = 6.6 m1

m1 = 0.1184 g

the mass of caffeine left in ( water + methylene chloride )

= 0.170 - 0.1184 = 0.0516 g (M2)

second extraction using same formula we can easily find out

now we take M2 = 0.0516 - m2

K = C1 /C2 =   (m/v)    /    (M/V)

4.6 = (m₂ / 5ml)    /    (0.0516 - m₂) / 10 ml  )or m *  10 ml / (0.0516 - m₂ ) *  5ml

0.0516 * 4.6 - 4.6 m₂ = 2 m₂

0.2373    = 6.6 m₂

m₂ = 0.0359 g

the mass of caffeine left in ( water + methylene chloride )

= 0.0516 - 0.0359 =   0.0157 g

Third extraction using same formula we can easily find out

third case take M2 = 0.0157 - m₃

K = C1 /C2 =   (m/v)    /    (M/v)

4.6 = (m₃ / 5 ml ) /    (0.0157 - m₃) / 10 ) or m *  10ml / (0.0157   - m₃ ) * 5ml

0.0157  *4.6 - 4.6 m₃ = 2 m₃

0.0722  = 6.6 m3

m3 = 0.0109 g

the total caffeine extracted in the given solution is

= m₁ + m₂ + m₃ = 0.1184 g +0.0359 g + 0.0109 g

m= 0.1652 g

so the toatl amount of caffeine of the solution is m= 0.1652 g