A student, who has not learned properly how to prepare solutions, added 51.3 g o
ID: 786164 • Letter: A
Question
A student, who has not learned properly how to prepare solutions, added 51.3 g of sucrose (FW = 342.31) to exactly 150.0 mL of dH2O and assumed that a 1.0 M solution had been prepared. The total solution volume increased to 180.0 mL after the sucrose was dissolved and the solution was thoroughly mixed. Determine the actual concentration of the sucrose solution prepared by the student. What did the student do wrong? If the solution had been prepared correctly to the desired 1.0 M concentration, what volume of this stock should be diluted to prepare 100.0 mL of a 1.0 mM working solution?
Explanation / Answer
number of moles of sucrose added = 51.3/342.31 = 0.149 moles
total volume V= 180 ml = 0.18 L
actual concentration of solution prepared = moles/volume = 0.149/0.18 = 0.82 M
The mistake he did is he has taken volume of water and weight of sucrose in wrong proportions.
suppose 1.0 M solution was made
100 mL of 1.0 mM solution is to be made
M1= 1 V1=?
M2= 1mM = 0.001 M V2 = 100 ml
M1 x V1 = M2 x V2
therefore V1 = M2xV2/M1 = 0.1 mL
therefore 0.1 mL of 1.0 M solution is required to prepare 100 mL of 1.0 mM solution
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