Amount and Concentration: Making and Diluting Solutions

1.Making solutions: % w/v

You have a very precious sample of purified enzyme and need to make a 0.5 mL solution at a concentration of 0.1 % w/v. How much do you need to weigh out?
Express your answer in mg to one significant figure. mg

2.Making solutions

Concentrated solutions of ethanol in water are commonly used to sterilise surfaces. The concentration of ethanol that is used is 70 % v/v. You need to fill a 500 mL sprayer bottle. How much ethanol and water will you need?
volume of ethanol = mL
volume of water = mL
 

3.Making solutions: g/L

You have weighed out 2.5 mg of an enzyme and need to make a 10 mg/ml solution. How much water do you need to dissolve it in?
ml

 4.Question:
a) Explain how to prepare 25 liters of a 0.10 M BaCl₂ solution, starting with solid BaCl₂.
b) Specify the volume of the solution in (a) needed to get 0.020 mol of BaCl₂.
Solution:
Part a): Molarity is an expression of the moles of solute per liter of solution, which can be written:
molarity (M) = moles solute / liters solution
Solve this equation for moles solute:
moles solute = molarity × liters solution
Enter the values for this problem:
moles BaCl₂ = 0.10 mol/liter × 25 liter
moles BaCl₂ = 2.5 mol
To determine how many grams of BaCl₂ are needed, calculate the weight per mole. Look up the atomic masses for the elements in BaCl₂ from the Periodic Table. The atomic masses are found to be:
Ba = 137
Cl = 35.5
Using these values:
1 mol BaCl₂ weighs 137 g + 2(35.5 g) = 208 g
So the mass of BaCl₂ in 2.5 mol is:
mass of 2.5 moles of BaCl₂ = 2.5 mol × 208 g / 1 mol
mass of 2.5 moles of BaCl₂ = 520 g
To make the solution, weigh out 520 g of BaCl₂ and add water to get 25 liters.
Part b): Rearrange the equation for molarity to get:
liters of solution = moles solute / molarity
In this case:
liters solution = moles BaCl₂ / molarity BaCl₂
liters solution = 0.020 mol / 0.10 mol/liter
liters solution = 0.20 liter or 200 cm³
Answer
Part a). Weigh out 520 g of BaCl₂. Stir in sufficient water to give a final volume of 25 liters.
Part b). 0.20 liter or 200 cm³



5.Prepare 1 liter of 1.00 M NaCl solution.
First calculate the molar mass of NaCl which is the mass of a mole of Na plus the mass of a mole of Cl or 22.99 + 35.45 = 58.44 g/mol

1. Weigh out 58.44 g NaCl.
2. Place the NaCl in a 1 liter volumetric flask.
3. Add a small volume of distilled, deionized water to dissolve the salt.
4. Fill the flask to the 1 L line.

If a different molarity is required, then multiply that number times the molar mass of NaCl. For example, if you wanted a 0.5 M solution, you would use 0.5 x 58.44 g/mol of NaCl in 1 L of solution or 29.22 g of NaCl.