How to Use This Molarity Calculator
- Solute: type a formula and the calculator finds the molar mass from the AP periodic table, or type the molar mass yourself.
- Two of three: enter any two of mass, volume, and molarity and leave the third blank.
- Significant figures: the answer is rounded to the least precise value you typed. Write 250. with a decimal point to count both zeros.
The Molarity Formula and Its Rearrangements
Molarity (M) = moles of solute ÷ liters of solution. It is the only concentration unit the AP exam asks you to calculate. The three rearrangements cover almost every problem: moles = M × L, liters = moles ÷ M, and grams = M × L × molar mass. Note that the volume is of the whole solution, not of the water added. The reasoning, with worked problems, is in AP Chemistry molarity; diluting a solution you already have uses M1V1 = M2V2 in the dilution calculator.
How Ion Concentrations Follow From Molarity
A soluble ionic compound, strong acid, or strong base breaks apart completely in water, so each ion’s concentration is the compound’s molarity times that ion’s subscript. In 0.100 M Na2SO4, [Na+] = 0.200 M and [SO42−] = 0.100 M; in 0.100 M Al2(SO4)3, [Al3+] = 0.200 M and [SO42−] = 0.300 M. Molecular solutes such as glucose and weak electrolytes such as NH3 do not dissociate completely, so the calculator does not list ions for them. Which ions matter in a reaction is covered in net ionic equations.
How to Find Molarity After Mixing Two Solutions
Find the moles of solute in each solution, add them, and divide by the combined volume. Averaging the two molarities only works when the volumes are equal. If the two solutions react instead, as an acid and a base do, use the balanced equation and stoichiometry instead of simple mixing. To see how solution calculations feed into an exam score, try the AP Chem Score Calculator.
Related Resources
- AP Chem Score Calculator
- All AP Chemistry Calculators
- AP Chemistry Molarity
- Dilution Calculator
- Molar Mass Calculator
- AP Chemistry Solutions
- Unit 3 Review: Properties of Substances and Mixtures
Frequently Asked Questions
How do you calculate molarity?
Divide the moles of solute by the liters of solution: M = mol / L. If you start from grams, convert to moles with the molar mass first. Dissolving 7.10 g of Na2SO4 (142.04 g/mol) to make 500. mL of solution gives 0.0500 mol / 0.500 L = 0.100 M.
How many grams do I need to make a solution of a given molarity?
Multiply molarity by liters to get moles, then moles by molar mass to get grams. For 250. mL of 0.500 M NaOH: (0.500 mol/L)(0.250 L) = 0.125 mol, and 0.125 mol x 40.00 g/mol = 5.00 g.
How do you find the concentration of each ion in a solution?
Multiply the molarity of the compound by the number of each ion in its formula, as long as it is a strong electrolyte that dissociates completely. In 0.100 M Na2SO4, [Na+] = 0.200 M and [SO4 2-] = 0.100 M.
What is the molarity when two solutions are mixed?
Add the moles of solute from each solution and divide by the total volume. Mixing 100. mL of 0.200 M with 50.0 mL of 0.500 M of the same solute gives (0.0200 + 0.0250) mol / 0.150 L = 0.300 M, assuming the volumes add.
Is molarity the same as molality?
No. Molarity is moles per liter of solution; molality is moles per kilogram of solvent. AP Chemistry uses only molarity, and molality calculations are not assessed on the exam.
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