Solution Chemistry Calculator: Molarity, Dilution and pH

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how much to weigh to prepare a solution

How much to weigh to prepare a solution (hydrate, purity and concentrated reagent)

Preparing a solution means multiplying three numbers and dividing by a fourth: concentration, final volume, molar mass and purity. What usually goes wrong is the hydrate molar mass and a forgotten purity, and that is where the tool helps.

The preparation calculation and the traps that change the result

First bring the target to mol/L of the reference species: g/L divides by the molar mass, mg/L divides by a thousand and then by the molar mass, and % w/v (g per 100 mL) multiplies by ten before dividing. Then n = c × V and m = n × M. For 250 mL of 0.1 mol/L sodium carbonate: n = 0.1 × 0.250 = 0.0250 mol and m = 0.0250 × 105.99 = 2.650 g. For 1 L of 0.02 mol/L potassium permanganate: 0.02 × 158.03 = 3.161 g.
Hydrates weigh their water of crystallization. Copper sulfate pentahydrate, CuSO4.5H2O, has M = 159.60 + 5 × 18.015 = 249.68 g/mol; for 500 mL at 0.1 mol/L you weigh 0.1 × 0.5 × 249.68 = 12.48 g. Using 159.60 g/mol would give 7.98 g, a solution 36 % too weak. When the concentration is requested in g/L of anhydrous salt but the bottle is the hydrate, multiply by 249.68 ÷ 159.60: 10 g/L of anhydrous CuSO4 calls for 15.64 g of hydrate per liter. The tool offers that choice when it detects a hydrate.
Purity enters as a divisor. Sodium hydroxide pellets are usually 97 % to 99 % pure and absorb water and CO2 from air, so the weighing is only approximate and the solution is standardized afterwards against a primary standard. For 500 mL of 1 mol/L NaOH at 97 %: 0.5 × 39.997 = 20.00 g pure and 20.00 ÷ 0.97 = 20.62 g from the bottle. If the reagent is a concentrated liquid, you measure a volume instead of weighing: for 500 mL of 1 mol/L HCl from 37 % (12.08 mol/L), V1 = 1 × 500 ÷ 12.08 = 41.4 mL. Always acid into water, slowly and with protection.

Masses to weigh, with hydrate and purity

CuSO4.5H2O 0.1 mol/L, 500 mL

Input: CuSO4.5H2O, 0.1 mol/L, 500 mL
Expected output: 12.48 g

M = 249.68 g/mol; 0.05 mol × 249.68.

Na2CO3 0.1 mol/L, 250 mL

Input: Na2CO3, 0.1 mol/L, 250 mL
Expected output: 2.650 g

M = 105.99 g/mol; 0.0250 mol.

KMnO4 0.02 mol/L, 1 L

Input: KMnO4, 0.02 mol/L, 1 L
Expected output: 3.161 g

M = 158.03 g/mol.

NaOH 1 mol/L, 500 mL, 97 %

Input: NaOH, 1 mol/L, 500 mL, pureza 97 %
Expected output: 20.62 g

20.00 g pure ÷ 0.97.

Anhydrous CuSO4 10 g/L, 1 L, weighing the hydrate

Input: CuSO4.5H2O, 10 g/L de CuSO4, 1 L
Expected output: 15.64 g

10 × 249.68 ÷ 159.60.

HCl 1 mol/L, 500 mL, from 37 %

Input: HCl 37 %, d = 1.19, 1 mol/L, 500 mL
Expected output: 41.4 mL

Top up with water to 500 mL.

Full tool FAQ

How do I calculate the molarity of a solution?

Molarity is moles of solute per liter of solution: c = m ÷ (M × V). For 5.844 g of NaCl (58.44 g/mol) in 1 L of solution: 5.844 ÷ 58.44 = 0.1000 mol, so 0.1000 mol/L. V is the final solution volume, measured to the mark of the flask, not the volume of water used.

What is the difference between molarity and molality?

How do I convert % w/w to mol/L?

How much should I weigh to prepare a solution, with hydrate and purity?

How do I use C1V1 = C2V2 and what does a 1:10 dilution mean?

How do I calculate the pH of a weak acid and when does the 5 % approximation hold?

Why is the pH of 1e-8 mol/L HCl not 8?

How does a buffer work and when does Henderson-Hasselbalch fail?

How do I read a titration curve and choose the indicator?

What are the limits of the calculations and what should the tool not be used for?

Frequently asked questions

Weigh directly in the volumetric flask?

No. Weigh in a beaker, dissolve in less solvent than the final volume, transfer while rinsing the beaker, and only then fill to the mark. The final volume is the flask volume, not the sum of solid and water.

How many significant figures should I weigh?

As many as the concentration precision you need and the balance allows. For 0.1000 mol/L, four figures (5.844 g of NaCl). The tool follows the fewest digits typed, with a floor of three.

Can I prepare a solution more concentrated than the reagent?

Not by dilution. The tool warns when the target is above the molarity of the concentrate (for example above 12.08 mol/L from 37 % HCl).

Next steps

The same solution in every unit
Unit	Value
Molarity (mol/L)	0.100 mol/L
Millimolarity (mmol/L)	100 mmol/L
Normality (eq/L)	0.100 eq/L
Molality (mol/kg)	0.101 mol/kg
Mole fraction (x)	0.00181
Mass concentration (g/L)	5.84 g/L
Milligrams per liter (mg/L)	5840 mg/L
% mass/volume (g/100 mL)	0.584 % m/v
% volume/volume	needs solute density
% mass/mass	0.584 % m/m
Mass fraction (titer)	0.00584
Parts per million (ppm, mg/kg)	5840 ppm
Parts per billion (ppb, µg/kg)	5.84 × 10⁶ ppb

The same solution in every unit

Unit

Value

Molarity (mol/L)

0.100 mol/L

Millimolarity (mmol/L)

100 mmol/L

Normality (eq/L)

0.100 eq/L

Molality (mol/kg)

0.101 mol/kg

Mole fraction (x)

0.00181

Mass concentration (g/L)

5.84 g/L

Milligrams per liter (mg/L)

5840 mg/L

% mass/volume (g/100 mL)

0.584 % m/v

% volume/volume

needs solute density

% mass/mass

0.584 % m/m

Mass fraction (titer)

0.00584

Parts per million (ppm, mg/kg)

5840 ppm

Parts per billion (ppb, µg/kg)

5.84 × 10⁶ ppb

The preparation calculation and the traps that change the result

First bring the target to mol/L of the reference species: g/L divides by the molar mass, mg/L divides by a thousand and then by the molar mass, and % w/v (g per 100 mL) multiplies by ten before dividing. Then n = c × V and m = n × M. For 250 mL of 0.1 mol/L sodium carbonate: n = 0.1 × 0.250 = 0.0250 mol and m = 0.0250 × 105.99 = 2.650 g. For 1 L of 0.02 mol/L potassium permanganate: 0.02 × 158.03 = 3.161 g.

Hydrates weigh their water of crystallization. Copper sulfate pentahydrate, CuSO4.5H2O, has M = 159.60 + 5 × 18.015 = 249.68 g/mol; for 500 mL at 0.1 mol/L you weigh 0.1 × 0.5 × 249.68 = 12.48 g. Using 159.60 g/mol would give 7.98 g, a solution 36 % too weak. When the concentration is requested in g/L of anhydrous salt but the bottle is the hydrate, multiply by 249.68 ÷ 159.60: 10 g/L of anhydrous CuSO4 calls for 15.64 g of hydrate per liter. The tool offers that choice when it detects a hydrate.

Purity enters as a divisor. Sodium hydroxide pellets are usually 97 % to 99 % pure and absorb water and CO2 from air, so the weighing is only approximate and the solution is standardized afterwards against a primary standard. For 500 mL of 1 mol/L NaOH at 97 %: 0.5 × 39.997 = 20.00 g pure and 20.00 ÷ 0.97 = 20.62 g from the bottle. If the reagent is a concentrated liquid, you measure a volume instead of weighing: for 500 mL of 1 mol/L HCl from 37 % (12.08 mol/L), V1 = 1 × 500 ÷ 12.08 = 41.4 mL. Always acid into water, slowly and with protection.

Masses to weigh, with hydrate and purity

CuSO4.5H2O 0.1 mol/L, 500 mL

Input: CuSO4.5H2O, 0.1 mol/L, 500 mL
Expected output: 12.48 g

M = 249.68 g/mol; 0.05 mol × 249.68.

Na2CO3 0.1 mol/L, 250 mL

Input: Na2CO3, 0.1 mol/L, 250 mL
Expected output: 2.650 g

M = 105.99 g/mol; 0.0250 mol.

KMnO4 0.02 mol/L, 1 L

Input: KMnO4, 0.02 mol/L, 1 L
Expected output: 3.161 g

M = 158.03 g/mol.

NaOH 1 mol/L, 500 mL, 97 %

Input: NaOH, 1 mol/L, 500 mL, pureza 97 %
Expected output: 20.62 g

20.00 g pure ÷ 0.97.

Anhydrous CuSO4 10 g/L, 1 L, weighing the hydrate

Input: CuSO4.5H2O, 10 g/L de CuSO4, 1 L
Expected output: 15.64 g

10 × 249.68 ÷ 159.60.

HCl 1 mol/L, 500 mL, from 37 %

Input: HCl 37 %, d = 1.19, 1 mol/L, 500 mL
Expected output: 41.4 mL

Top up with water to 500 mL.

Full tool FAQ

How do I calculate the molarity of a solution?

What is the difference between molarity and molality?

How do I convert % w/w to mol/L?

How much should I weigh to prepare a solution, with hydrate and purity?

How do I use C1V1 = C2V2 and what does a 1:10 dilution mean?

How do I calculate the pH of a weak acid and when does the 5 % approximation hold?

Why is the pH of 1e-8 mol/L HCl not 8?

How does a buffer work and when does Henderson-Hasselbalch fail?

How do I read a titration curve and choose the indicator?

What are the limits of the calculations and what should the tool not be used for?

Frequently asked questions

Weigh directly in the volumetric flask?

How many significant figures should I weigh?

As many as the concentration precision you need and the balance allows. For 0.1000 mol/L, four figures (5.844 g of NaCl). The tool follows the fewest digits typed, with a floor of three.

Can I prepare a solution more concentrated than the reagent?

Not by dilution. The tool warns when the target is above the molarity of the concentrate (for example above 12.08 mol/L from 37 % HCl).

The same solution in every unit
Unit	Value
Molarity (mol/L)	0.100 mol/L
Millimolarity (mmol/L)	100 mmol/L
Normality (eq/L)	0.100 eq/L
Molality (mol/kg)	0.101 mol/kg
Mole fraction (x)	0.00181
Mass concentration (g/L)	5.84 g/L
Milligrams per liter (mg/L)	5840 mg/L
% mass/volume (g/100 mL)	0.584 % m/v
% volume/volume	needs solute density
% mass/mass	0.584 % m/m
Mass fraction (titer)	0.00584
Parts per million (ppm, mg/kg)	5840 ppm
Parts per billion (ppb, µg/kg)	5.84 × 10⁶ ppb

The same solution in every unit

Unit

Value

Molarity (mol/L)

0.100 mol/L

Millimolarity (mmol/L)

100 mmol/L

Normality (eq/L)

0.100 eq/L

Molality (mol/kg)

0.101 mol/kg

Mole fraction (x)

0.00181

Mass concentration (g/L)

5.84 g/L

Milligrams per liter (mg/L)

5840 mg/L

% mass/volume (g/100 mL)

0.584 % m/v

% volume/volume

needs solute density

% mass/mass

0.584 % m/m

Mass fraction (titer)

0.00584

Parts per million (ppm, mg/kg)

5840 ppm

Parts per billion (ppb, µg/kg)

5.84 × 10⁶ ppb