Hi! I attended a UA workshop on Monday and I was just reviewing some questions.
Suppose that you have 105.0 grams of NaCl in 500.0 mL. If you take 300.0 mL of this solution, then
(a) How many mL of 0.25 M NaCl can you make?
(b) What will be the molarity of the solution when added to a 1000 mL flask filled with water?
The answers are 4300 ml for (a) and 1.08 M for (b), but how do we get there? I'm overall pretty confused about molarity/dilution problems if anyone could provide a walkthrough or some general tips!
Review Question [ENDORSED]
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Julie Tran 3C
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Re: Review Question [ENDORSED]
Hello,
As you know, both of these problems require manipulating these fundamental equations:
M = m/V
M1V1 = M2V2 (can be derived by observing that m = MV)
For problem (a):
1. This is asking you for "how many mL," meaning that the final value you are trying to determine is volume.
2. Thus we will use V = m/M
3. You are given the value of M (0.25M), so you just need to figure out how many moles of NaCl you have.
4. Since you took 300 mL of the original solution, all of the moles you have are contained in that 300 mL.
5. Since m = MV, find the molarity of the original solution, multiply it by 0.3L to find the moles, and plug it into V = m/M to find the final volume.
For problem (b):
1. This is asking you for "molarity", meaning that the final value you are trying to determine is molarity.
2. The scenario given is that of a dilution (we're adding water to the 0.25 M solution from problem (a).
3. Thus, we will use M1V1 = M2V2 and solve for M2 (the final molarity after the dilution).
4. Knowing that M1 = 0.25M, V1 = the volume you found in problem (a), and V2 = 1 L (1000mL), you can plug these values in and solve for M2.
In short, try to reverse-engineer the problem based on what you need and what the experimental context is.
Hope this helps!
As you know, both of these problems require manipulating these fundamental equations:
M = m/V
M1V1 = M2V2 (can be derived by observing that m = MV)
For problem (a):
1. This is asking you for "how many mL," meaning that the final value you are trying to determine is volume.
2. Thus we will use V = m/M
3. You are given the value of M (0.25M), so you just need to figure out how many moles of NaCl you have.
4. Since you took 300 mL of the original solution, all of the moles you have are contained in that 300 mL.
5. Since m = MV, find the molarity of the original solution, multiply it by 0.3L to find the moles, and plug it into V = m/M to find the final volume.
For problem (b):
1. This is asking you for "molarity", meaning that the final value you are trying to determine is molarity.
2. The scenario given is that of a dilution (we're adding water to the 0.25 M solution from problem (a).
3. Thus, we will use M1V1 = M2V2 and solve for M2 (the final molarity after the dilution).
4. Knowing that M1 = 0.25M, V1 = the volume you found in problem (a), and V2 = 1 L (1000mL), you can plug these values in and solve for M2.
In short, try to reverse-engineer the problem based on what you need and what the experimental context is.
Hope this helps!
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