6. Define the following units of concentration, in terms of base units. Molarity Molality parts per million Parts per billion 7. You have 100 mL of a 25.0 μM Naci solution. How many moles of NaCl are there? 8. Express the concentration in 7 in terms of ppm and ppb. 9. How would you make the solution in 7 from a stock solution of 0.500 M NaCI?
Chemists often use molarity M, in moles/liter, to measure the concentration of solutions. Molarity is a common unit of concentration because the volume of a liquid is very easy to measure. However, the drawback of using molarity is that volume is a temperature-dependent quantity. As temperature changes, density changes, which affects volume. Volume markings for most laboratory glassware are calibrated for room temperature, about 20âC.
Fortunately, there are several other ways of expressing concentration that do not involve volume and are therefore temperature independent.
A 2.350Ã10â2M solution of NaCl in water is at 20.0âC. The sample was created by dissolving a sample of NaCl in water and then bringing the volume up to 1.000 L. It was determined that the volume of water needed to do this was 999.4 mL . The density of water at 20.0âC 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.
mNaCl =
.0235m
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Your answer has units of mol/L of water. You need to convert the volume of water to kilograms using the density provided.
Part B
Calculate the mole fraction of salt in this solution.
Express the mole fraction to four significant figures.
ÏNaCl =
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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.
percent by mass NaCl =
%
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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.