Posted by Ana on .
I have these two questions on my section review that I don't know how to answer. Can someone help me? Please and thank you.
3. Describe in your own words how to prepare 100 mL of a .85 M solution of sodium chloride.
4. If you dissolve 2.00 mol KI in 1.00 L of water, will you get a 2.00 M solution? Explain your answer.
3. M = Molarity = mols per liter
0.85 M NaCl is equal to 0.85 mols/L
You have 100 ml.
(0.85 mol/L)(0.1 L) = 0.085 mol of NaCl
In other words, add 0.085 mols of NaCl
to the 100 ml solution.
OR add (0.085 mol NaCl) * (58.44 g
NaCl/molNaCl) = 4.9674 grams NaCL
Add 4.97 g NaCl to 100 ml solution.
4. Again, M = Molarity = moles/Liter
2.00 mol kl / 1.00 L = 2.00 M
Matt's instructions can be interpreted the wrong way so let me clarify the directions. Both questions pose the same risk in answering.
To prepare 100 mL of 0.85 M NaCl, add 4.9674 g NaCl to a 100 mL volumetric flask, add water, swirl to dissolve, when completely dissolved add water to the mark. This will give you 100 mL of SOLUTION as Matt points out. Note, however, that this is NOT the same thing as adding 4.9674 g NaCl to 100 mL water.
Next question. Will you get a 2.00 M solution by dissolving 2.00 moles KI in 1 L water. ABSOLUTELY NOT. (Actually, assuming the water has a density of 1 g/mL, you will get a 2.00 molal solution but not 2.00 molar). Why? Because you have more than a liter of solution. If you add 2 moles (whatever that is in grams) to 1000 mL of water, you will end up with something OVER 1000 mL and so it may be 1002 mL solution or something like that. When making a molar solution, one ALWAYS adds the mass of solute to a volumetric flask, add solvent, swirl to dissolve, and the final step is adding solvent to the mark of the volumetric flask.
Oh, alright, thank you so much for clarifying..