Calculate the concentrations of each of the following solutions.
(a) the molality of chloride ions in an aqueous solution of iron(III) chloride for which xFeCl3 is 0.0195
(b) the molality of hydroxide ions in a solution prepared from 9.16 g of barium hydroxide dissolved in 179 g of water
(c) the molality of 12.70 M NH3(aq) with a density of 0.9519 g·cm-3
I have repeatedly tried to figure out how to solve these, but I am getting something wrong. I know molality is mol solute/kg solvent. I'm especially having trouble with the second question. Please don't just give me the answer--try to explain how you solved it. An answer would be helpful so I can compare what I come up with though. Thanks!!
For a.
Assume you have 1 mole of solution; therefore, you will have 0.0195 moles FeCl3 and 0.9805 moles H2O.
0.9805 moles x molar mass = 17.663 g H2O and since molality = moles/kg solvent that will be 0.0195 moles/0.017663 = ??m. Round to 3 s.f.
Check my arithmetic.
Got it. for A i was using molar mass for FeCl3 instead of water..oops. I solved C...but I am still confused with the second question of anyone can help me out... Thanks!
the answer for a was actually incorrect
The answer to a is not incorrect if you re-read the problem. What you have calculated is molality of FeCl3, the problem asks for molality of Cl^-, so multiply the final answer you had by 3.
For #2, I think you reposted this later and I responded. If you still have questions or think there was an error, please repost a new post at the top of the page and I'll take a look at it.
Sure! I'd be happy to help you solve these problems.
(a) To find the molality of chloride ions in an aqueous solution of iron(III) chloride, we need to determine the moles of chloride ions and the mass of water.
Given:
- xFeCl3 = 0.0195 (moles of iron(III) chloride)
- We need the molality of chloride ions, so we need to find the moles of chloride ions.
In one mole of iron(III) chloride (FeCl3), there are three moles of chloride ions (Cl-). Therefore, the moles of Cl- in the solution would be 3 times the value of xFeCl3.
So, the moles of chloride ions (mol Cl-) = 3 * xFeCl3
Now, we need to find the mass of water (kg solvent) in the solution. Since the problem doesn't provide the mass directly, we need other information to calculate it.
(b) To find the molality of hydroxide ions in a solution of barium hydroxide, we need to determine the moles of hydroxide ions and the mass of water.
Given:
- Mass of barium hydroxide (Ba(OH)2) = 9.16 g
- Mass of water = 179 g
- We need the molality of hydroxide ions, so we need to find the moles of hydroxide ions.
First, we need to convert the mass of barium hydroxide to moles. To do this, divide the given mass by the molar mass of barium hydroxide.
Molar mass of Ba(OH)2 = atomic mass of Ba + 2*(atomic mass of O) + 2*(atomic mass of H)
After finding the moles of Ba(OH)2, multiply it by 2 to get the moles of hydroxide ions because there are two moles of OH- per mole of Ba(OH)2.
Now, we need to find the mass of water (kg solvent) in the solution. Since the problem provides the mass of water directly, you can use that value.
(c) To find the molality of an ammonia solution with a given molarity and density, we need to determine the moles of ammonia and the mass of water.
Given:
- Molarity of NH3(aq) = 12.70 M
- Density of NH3(aq) = 0.9519 g·cm-3
- We need the molality of the solution, so we need to find the moles of ammonia.
First, convert the given concentration from moles per liter (Molarity, M) to moles per kilogram (molality, m) using the density of the solution.
Moles of NH3 = Molarity * Volume of solution (in liters)
To find the volume of the solution, divide the mass of the solution by its density.
Volume of solution = Mass of solution / Density of solution
Finally, to determine the overall mass of the solution (kg solvent), subtract the mass of NH3 from the mass of the solution.
Mass of water (kg solvent) = Mass of solution - Mass of NH3
Now, use the moles of NH3 and the mass of water to calculate the molality using the formula:
molality (m) = moles of NH3 / mass of water
I hope this explanation helps you understand how to solve these problems. Let me know if you have any further questions!