Molality Problems: Show work if possible! Will mark Brainliest if correct! (NEED ASAP!) tysm in advance :)

1. What is the molality of a solution in which 0.135 moles of CO2 is dissolved in 490mL of water? (1kg of water = 1L of water)

2. What is the molality of a solution in which 23g of NaCl are dissolved in 250g of water?

3. How many moles of HC1 are needed to prepare a 3m (3molal) solution with 1.5Kg of water?

1. 0.27551 m

2.  1.574 m

3. 4.5kg

Explanation:

Molality or molal concentration shows how many moles of molecules dissolved for every one kilogram of solvent. Molality uses lower case m as a unit. You might confuse it with molarity, which divides using volume(one liter) instead of mass(one kilogram).  

Molality is preferred in many cases because the volume will expand when the system temperature increases, while the mass is still the same.

1.  

We have the moles of the solute (0.135 mol of CO2) and the volume of the solvent. We still need to find the mass of the solvent. If the mass of water 1kg/L then the mass will be: 490 mL * (1L/ 1000ml) * (1 kg/ L)= 0.49 kg.

The molality of the solution will be:

m=  mol of solute/mass of solvent

m=0.135 mol/ 0.49 kg  

m= 0.27551 m

2.  

We have the mass of the molecule (23g) and the mass of the solvent(250g). Remember that we need the moles of the molecule, not the mass, so we need to convert it. The unit of the mass is grams too, not kilograms. The molecular mass of NaCl is 58.44 g/mol, so the moles will be: 23/ (58.44)= 0.3935 moles

The molality of the solution will be:  

m= mol of solute/mass of solvent

m=0.3935 mol/ 250g  * ( 1000g/1kg)

m= 1.574 m

3.  

In this question, we have the target molality(m=3) and the mass of the solvent(mass= 1.5kg). Every variable is using the correct units, so we don't have to convert anything. If we want to find out the moles of the molecule needed, we just put the number into the formula.

The calculation will be:

m= mol of solute/mass of solvent

3m= mol of solute/ 1.5kg

mol of solute=(3mol/kg)* 1.5kg

mol of solute= 4.5kg