Weight/Volume Percentage Concentration (w/v %) Chemistry Tutorial

⚛ Weight/Volume Percentage Concentration is a measurement of the concentration of a soluton.

· Weight/Volume percentage concentration is also known as mass/volume percentage concentration.

⚛ weight/volume percentage concentration is usually abbreviated as w/v (%) or w/v% or (w/v)% or %(w/v) or %w/v

· mass/volume percentage concentration is usually abbreviated as m/v (%) or m/v% or (m/v)% or %(m/v) or %m/v

⚛ w/v% (m/v%) is a useful concentration measure when dispensing reagents.

⚛ To calculate w/v% concentration (m/v% concentration):

⚛ Common units⁽¹⁾ for w/v% concentration are g/100 mL (grams of solute per 100 mL of solution)

· example: 5%(w/v) = 5 g/100 mL

· example: 12%(m/v) = 12 g/100 mL

⚛ Rearrange the equation for w/v% (m/v%) concentration to find:

(i) mass of solute

mass(solute) = [volume(solution) × (w/v)%]/100

(ii) volume of solution

volume(solution) = [mass(solute)/(w/v)%] × 100

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Weight/Volume percentage concentration (w/v%), or mass/volume percentage concentration (m/v%), is a measure of the concentration of a solution.
w/v% or m/v% is calculated by dividing the mass of the solute in grams by the volume of solution in millilitres then multiplying this by 100 as shown below.

A percentage concentration tells us how many parts of solute are present per 100 parts of solution. In weight per volume terms (mass per volume terms), this means a percentage concentration tells us the parts of solute by mass per 100 parts by volume of solution.

In SI units, w/v% (m/v%) concentration would be given in kg/100 L, but these units are far too large to be useful to Chemists in the lab, grams and milliltres are more convenient units for us.

Recall that 1 kg = 1 000 g

Recall that 1 L = 1 000 mL

so kg/100 L = 1000 g/100 000 mL = g/100 mL

so the units for w/v% concentration are most often given as g/100 mL

Therefore the units for w/v% (m/v%) concentration are grams of solute per 100 mL of solution (g/100 mL).

This means that the weight/volume percentage concentration (mass/volume percentage concentration) can be given in different, but equivalent, ways. Some examples are given in the table below:

To prepare a solution with a particular concentration, you will weigh out the solid and then dissolve it enough solvent to make a known volume of solution. Therefore you will know the value of two quantities:

• mass of solute
• volume of solution

which you can use to calculate the weight/volume percentage concentration (mass/volume percentage concentration).

To calculate a weight/volume percentage concentration (mass/volume percentage concentration):

1. Step 1: Write the equation: either w/v% = w/v × 100 or m/v% = m/v × 100
2. Step 2: Identify the solute and solvent (by name or chemical formula)
3. Step 3: Extract the data from the question (mass of solute, volume of solution)
4. Step 4: Check the units for consistency and convert if necessary (mass in grams, volume in millilitres)
5. Step 5: Substitute these values into the equation and solve.
6. Step 6: Write the answer

Worked Examples: w/v% (m/v%) Calculations

Solution:

Step 1: Write the equation: either w/v% = w/v × 100 or m/v% = m/v × 100

weight/volume (%) = (mass solute ÷ volume of solution) × 100

Step 2: Identify the solute and solvent (by name or chemical formula)

solute = sodium chloride = NaCl

solvent is water, H₂O, because this is an aqueous solution.

Step 3: Extract the data from the question (mass of solute, volume of solution)

mass solute (NaCl) = 5 g

volume of solution = 250 mL

Step 4: Check the units for consistency and convert if necessary (mass in grams, volume in millilitres)

mass solute (NaCl) = 5 g (no unit conversion needed)

volume of solution = 250 mL (no unit conversion needed)

Step 5: Substitute these values into the equation and solve.

w/v (%) = (5 g ÷ 250 mL) × 100 = 2 g/100 mL

(Note: only 1 significant figure is justified)

Step 6: Write the answer

w/v% = 2 g/100 mL = 2%(w/v) = 2%(m/v)

Solution:

Step 1: Write the equation: either m/v% = m/v × 100 or w/v% = w/v × 100

mass/volume (%) = (mass solute ÷ volume of solution) × 100

Step 2: Identify the solute and solvent (by name or chemical formula)

solute = barium chloride = BaCl₂

solvent = water = H₂O_(l) (This is an aqueous solution)

Step 3: Extract the data from the question (mass of solute, volume of solution)

mass of solute = mass(BaCl₂) = 10.00 g

mass of solvent = mass(H₂O) = 90.00 g

density of solution = 1.090 g/mL

Step 4: Check the units for consistency and convert if necessary (mass in grams, volume in millilitres)

mass of solute = mass(BaCl₂) = 10.00 g (no unit conversion needed)

volume of solution is unknown and needs to be calculated using the density of solution and the masses of solute and solvent:

density = mass(solution) ÷ volume(solution)

mass(solution) = mass(solute) + mass(solvent)

mass(solution) = 10.00 g BaCl₂ + 90.00 g water = 100.0 g

volume(solution) = mass(solution) ÷ density(solution)

volume(solution) = 100.0 g ÷ 1.090 g/mL = 91.74 mL

Step 5: Substitute these values into the equation and solve.

m/v (%) = (mass solute ÷ volume solution) × 100

m/v (%) = (10.00 g ÷ 91.74 mL) × 100

m/v (%) = 10.90 g/100 mL

(Note: only 4 significant figures are justified)

Step 6: Write the answer

m/v% = 10.90 g/100 mL = 10.90 %(m/v) = 10.90 %(w/v)

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The most common units for w/v% (m/v%) concentration are g/100 mL (grams of solute per 100 mL of solution).
If the mass of the solute is not given in grams then you will need to convert the units to grams.
If the volume of the solution is not given in millilitres then you will need to convert the units to millilitres.

Worked Examples: w/v% (m/v%) Calculations Requiring Unit Conversions

Solution:

Step 1: Write the equation: either w/v% = w/v × 100 or m/v% = m/v × 100

Step 2: Identify the solute and solvent (by name or chemical formula)

solute = potassium chloride = KCl

solvent is water, H₂O, because this is an aqueous solution.

Step 3: Extract the data from the question (mass of solute, volume of solution)

mass KCl = 45.0 g

volume of solution = 2.00 L

Step 4: Check the units for consistency and convert if necessary (mass in grams, volume in millilitres)

mass KCl = 45.0 g (mass in g, no unit conversion needed)

volume of solution = 2.00 L (need to convert to mL)
V(solution) = 2.00 L × 10³ mL/L = 2000 mL

Step 5: Substitute these values into the equation and solve.

w/v (%) = [mass solute (g) ÷ volume solution (mL)] × 100

w/v (%) = [45.0 g ÷ 2000 mL] × 100 = 2.25 g/100 mL

(Note: only 3 significant figures are justified)

Step 6: Write the answer

w/v% = 2.25 g/100 mL = 2.25 %(w/v) = 2.25 %(m/v)

Solution:

Step 1: Write the equation: either m/v% = m/v × 100 or w/v% = w/v × 100

Step 2: Identify the solute and solvent (by name or chemical formula)

solute = sucrose

solvent is water, H₂O, because this is an aqueous solution.

Step 3: Extract the data from the question (mass of solute, volume of solution)

mass solute (sucrose) = 750 mg

volume solution = 15 mL

Step 4: Check the units for consistency and convert if necessary (mass in grams, volume in millilitres)

mass solute (sucrose) = 750 mg (need to convert to grams)
mass solute = 750 mg ÷ 1000 mg/g = 0.750 g

volume solution = 15 mL (no conversion needed)

Step 5: Substitute these values into the equation and solve.

w/v (%) = [mass solute (g) ÷ volume solution (mL)] × 100

w/v (%) = (0.750 g ÷ 15 mL) × 100 = 5.0 g/100 mL

(Note: only 2 significant figures are justified)

Step 6: Write the answer

m/v% = 5.0 g/100 mL = 5.0 %(m/v) = 5.0 %(w/v)

Solution:

Step 1: Write the equation: either w/v% = w/v × 100 or m/v% = m/v × 100

Step 2: Identify the solute and solvent (by name or chemical formula)

solute = sodium hydroxide = NaOH

solvent is water, H₂O, because this is an aqueous solution.

Step 3: Extract the data from the question (mass of solute, volume of solution)

mass solute (NaOH) = 1.15 kg

volume solution = 186.4 L

Step 4: Check the units for consistency and convert if necessary (mass in grams, volume in millilitres)⁽²⁾

mass solute (NaOH) = 1.15 kg (convert units to g)

mass solute (NaOH) = 1.15 kg = 1.15 kg × 1000 g/kg = 1 150 g

volume solution = 186.4 L (convert units to mL)

volume solution = 186.4 L = 186.4 L × 1000 mL/L = 186 400 mL

Step 5: Substitute these values into the equation and solve.

w/v (%) = [mass solute (g) ÷ volume solution (mL)] × 100

w/v (%) = (1 150 g ÷ 186 400 mL) × 100

w/v (%) = 0.617 g/100 mL

(Note: only 3 significant figures are justified)

Step 6: Write the answer

w/v% = 0.617 g/100 mL = 0.617 %(w/v) = 0.617 %(m/v)

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In the sections above we calculated the w/v% concentration of solutions using the known mass of solute and volume of solution.

When we come to use this solution in the lab, we are most likely to use a pipette or burette to deliver a volume of solution.

If we know the volume of solution used, we can calculate the mass of solute present.

If we know the mass of solute we want to use, we can calculate the volume of solution we will need to use.

Solution:

Step 1: Write the equation: either w/v% = w/v × 100 or m/v% = m/v × 100

Rearrange the equation to find volume of solution:

w/v% = mass solute (g) ÷ volume solution (mL) × 100

Multiply both sides of the equation by volume

w/v% × volume (mL) = (mass solute (g) ÷ volume solution (mL)) × volume (mL) × 100

w/v% × volume (mL) = mass solute (g) × 100

Divide both sides of the equation by w/v%

[w/v% × volume (mL)]/w/v% = (mass solute (g) × 100)/w/v%

Step 2: Identify the solute and solvent (by name or chemical formula)

solute = sodium chloride = NaCl

solvent is water, H₂O, because this is an aqueous solution.

Step 3: Extract the data from the question (mass of solute, volume of solution)

mass of solute required = mass(NaCl) = 1.22 g

concentration of NaCl_(aq) provided = w/v (%) = 11.78 g/100 mL

volume of solution needed = ? mL

Step 4: Check the units for consistency and convert if necessary (mass in grams, volume in millilitres)

mass of solute required = mass(NaCl) = 1.22 g (no unit conversion needed)

concentration of NaCl_(aq) provided = w/v (%) = 11.78 g/100 mL (no unit conversion needed)

volume of solution needed = ? mL

Step 5: Substitute these values into the equation and solve.

(Note: only 3 significant figures are justified)

Step 6: Write the answer

volume of solution = 10.4 mL

Solution:

Step 1: Write the equation: either w/v% = w/v × 100 or m/v% = m/v × 100

Rearrange the equation to find mass of solute:

w/v(%) = mass solute (g) ÷ volume solution (mL) × 100

Divide both sides of the equation by 100

w/v(%) ÷ 100 = mass solute (g) ÷ volume solution (mL)

Multiple both sides of the equation by volume solution (mL)

mass solute (g) = (w/v (%) ÷ 100) × volume solution (mL)

Step 2: Identify the solute and solvent (by name or chemical formula)

solute = potassium iodide = KCl

solvent is water, H₂O, because this is an aqueous solution.

Step 3: Extract the data from the question (mass of solute, volume of solution)

mass solute = mass(KI) = ? g

volume(KI_(aq)) = 14.86 mL

concentration of solution = w/v%(KI_(aq)) = 32.44 g/100 mL

Step 4: Check the units for consistency and convert if necessary (mass in grams, volume in millilitres)

volume(KI_(aq)) = 14.86 mL (no unit conversion needed)

w/v%(KI_(aq)) = 32.44 g/100 mL (no unit conversion needed)

Step 5: Substitute these values into the equation and solve.

mass solute (g) = (w/v% ÷ 100) × volume solution (mL)

mass(KI) = (32.44 ÷ 100) × 14.86 = 4.821 g

(Note: 4 significant figures are justified)

Step 6: Write the answer

mass of solute (KI) = 4.821 g

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Determine the mass in grams of potassium nitrate (KNO₃) in 22.65 mL of a 2.15%(m/v) aqueous solution of potassium nitrate.

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