What is the molality of a solution containing 7.8 g of MgCl2 in 725 g of water?
mols MgCl2 = grams/molar mass
Then m = mols/kg solvent.
To find the molality of a solution, you need to know the mass of the solute (in this case, MgCl2) and the mass of the solvent (in this case, water).
Step 1: Calculate the moles of MgCl2
First, you need to convert the mass of MgCl2 from grams to moles. To do this, you can use the molar mass of MgCl2. The molar mass of Mg is 24.31 g/mol, and the molar mass of Cl is 35.45 g/mol.
Molar mass of MgCl2 = (1 * molar mass of Mg) + (2 * molar mass of Cl)
= (1 * 24.31 g/mol) + (2 * 35.45 g/mol)
= 95.21 g/mol
Now, you can calculate the moles of MgCl2 using the formula:
moles = mass / molar mass
moles of MgCl2 = 7.8 g / 95.21 g/mol
Step 2: Calculate the moles of water
The mass of water given is 725 g. To calculate the moles of water, you need to know the molar mass of water, which is approximately 18.02 g/mol.
moles of water = 725 g / 18.02 g/mol
Step 3: Calculate the molality
The molality (m) of a solution is defined as the number of moles of solute per kilogram of solvent. To calculate molality, you divide the moles of solute by the mass of the solvent in kilograms.
molality = moles of solute / mass of solvent (in kg)
mass of solvent (in kg) = mass of water / 1000
Now you can plug in the values to calculate the molality:
molality = moles of MgCl2 / (mass of water / 1000)
Thus, by substituting the calculated values into the equation, you can find the molality.