A 2.700×10 2 M solution of NaCl in water is at 20.0C. The sample was created by

Question

A 2.700×102M solution of NaCl in water is at 20.0C. The sample was created by dissolving a sample of NaClin water and then bringing the volume up to 1.000 L. It was determined that the volume of water needed to do this was 999.3 mL . The density of water at 20.0C is 0.9982 g/mL.

Part A

Calculate the molality of the salt solution.

Express your answer to four significant figures and include the appropriate units. ( I tried 2.70 and 2.71 and got it wrong)

Part B

Calculate the mole fraction of salt in this solution.

Express the mole fraction to four significant figures.

Part C

Calculate the concentration of the salt solution in percent by mass.

Express your answer to four significant figures and include the appropriate units.

Part D

Calculate the concentration of the salt solution in parts per million.

Express your answer as an integer to four significant figures and include the appropriate units.

Explanation / Answer

SOLUTION:

(A) The salt solution is 2.700×102 M. It means 2.700×102 of NaCl is dissolven in 1L of solution.

molality = (no. of moles of salt / mass of solvent) X 1000

Mass of Solvent = density X volume = 0.9982 g/mL X 1000mL = 998.2g

molality = (2.700×102 / 998.2) X 1000 = 0.02704 m = 2.704 X 102 m

(B) Mole fraction of the salt (xNaCl) = moles of salt (nNaCl) / moles of salt (nNaCl) + moles of water (nH2O)

moles of water = mass of water / molar mass = 998.2 / 18.01 = 55.42

Mole fraction of the salt (xNaCl) = 2.700 X 102 / 2.700 X 102 + 55.42 = 4.869 X 10-4

(C) Pecentage is eather expressed as maas/volume or mass/mass, but you have not mentioned which one you want. Here we calculate mass/volume percent.

Percentage of NaCl (mass/Volume) = (Mass of salt / volume of solution ) X 100

Mass of salt = number of moles NaCl X molar mass of NaCl

Mass of salt = 2.700 X 102 X 55.44 = 1.4967g

Percentage of NaCl (mass/Volume) = (1.4967 g/ 1000mL) X 100 = 0.14967 percent = 0.1497 % (mass/Volume)

(D)

ppm(NaCl) = (Mass of NaCl / (total mass of the solution) X 106 = (1.4967 / 1.4967 + 998.2) X 106 = 1497ppm

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