You have 120.0 mL of a 0.245 M solution of NaCl in an open beaker. After several days, you

test the solution and find that it now has a concentration of 0.348 M. How much water must
have evaporated from the beaker given the new concentration?

So how many mols did you have to start? That's mols = M x L = 0.120 x 0.245 = 0.0294.

When some of the water has evaporated you know you still have 0.0294 mols there. And the molarity now is 0.348M and not 0.245M. So M = mols/L. M = 0.348; mols = 0.0294. Solve for L of solution and subtract from the starting volume to find how much has evaporated.

Dear Dr Bob222,

Thank you so much! The answer is 35.5 mL on H2O has evaporated.

To find out how much water must have evaporated from the beaker, we need to calculate the initial amount of solute (NaCl) and compare it with the final amount.

Step 1: Calculate the initial amount of solute (NaCl):
Initial concentration = 0.245 M
Initial volume = 120.0 mL

Using the formula:
Initial amount of solute (NaCl) = Initial concentration x Initial volume

Initial amount of solute (NaCl) = 0.245 M x 120.0 mL

Step 2: Calculate the final amount of solute (NaCl):
Final concentration = 0.348 M

Since the final volume is not given, we can assume it remains the same as the initial volume (no water added or removed). Therefore, the final volume is also 120.0 mL.

Using the formula:
Final amount of solute (NaCl) = Final concentration x Final volume

Final amount of solute (NaCl) = 0.348 M x 120.0 mL

Step 3: Calculate the difference in the amount of solute:
Difference in amount of solute = Final amount of solute - Initial amount of solute

Difference in amount of solute = (0.348 M x 120.0 mL) - (0.245 M x 120.0 mL)

Step 4: Calculate the volume of water evaporated:
Since the solute is sodium chloride (NaCl), we assume its molar mass to be 58.44 g/mol.

Using the formula:
Volume of water evaporated = Difference in amount of solute x (molar mass of water / molar mass of NaCl)

The molar mass of water (H2O) is 18.015 g/mol.

Substituting the values:
Volume of water evaporated = Difference in amount of solute x (18.015 g/mol / 58.44 g/mol)

Now, let's perform the calculations to find the volume of water evaporated.

To solve this problem, we can use the concept of dilution.

First, let's understand the formula for dilution:

C1V1 = C2V2

Where:
C1 = initial concentration
V1 = initial volume
C2 = final concentration
V2 = final volume

In this case, we have an initial concentration (C1) of 0.245 M, an initial volume (V1) of 120.0 mL, and a final concentration (C2) of 0.348 M. We need to find the change in volume, which represents the water that has evaporated.

Using the formula for dilution, we can rearrange it to solve for V2:

V2 = (C1V1) / C2

Let's substitute the given values into the equation:

V2 = (0.245 M * 120.0 mL) / 0.348 M

Now, we can calculate V2:

V2 = 84.136 mL

So, according to the equation, the final volume (V2) is 84.136 mL. We started with 120.0 mL and ended up with 84.136 mL. The difference represents the volume of water that has evaporated from the beaker, which is:

120.0 mL - 84.136 mL = 35.864 mL

Therefore, approximately 35.864 mL of water must have evaporated from the beaker given the new concentration.