A 15.0-mL sample of NaCl solution has a mass of 15.78 g. Afterthe NaCl solution

Question

A 15.0-mL sample of NaCl solution has a mass of 15.78 g. Afterthe NaCl solution is evaporated to dryness, the dry salt residue has a mass of 4.69 g. Calculate the following concentrations forthe NaCl solution.

a. % (m/m)

b. % (m/v)

c. molarity

These are the results i got:

a. 29.7% but i am not sure if it is correct

b. 31.3%

c. 5.35 M Nacl solution

On the % (m/m) i have a question; based on the formula:

% (m/m)= mass of solute/mass of solution X100

or

% (m/m)= grams of solute/ grams of solute + grams solvent X100

Which one should i used in this problem? I will appreciate ur help!

Explanation / Answer

NaCl mass = 4.69 g

solution mass = 15.78 g

a) m/m% = ( 100 x NaCl mass/ solution mass) = ( 100 x 4.69 /15.78) = 29.72 %

b) m/v % = ( 100 x NaCl mass in g / solution vol in ml) = ( 100 x 4.69 /15) = 31.27 %

c) molarity = moles of NaCl / solution vol in L

moles of NaCl = mass/ molar mass of NaCl = 4.69 /58.44 = 0.08025

solution vol = 15 ml = 0.015 L

Molarity = 0.08025/0.015 = 5.35 M

solution mass = solute + solvent mass , hence when we were given solution mass which is 15.78 g we use it directly , or esle we add solute and solvent mass and find solution mass

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