Water of Crystallization Calculator: % Water in a Hydrate and Its Formula
Find the percentage of water of crystallization in a hydrate ·nH₂O from its formula, work out n and the hydrate formula from lab masses before and after heating (with or without the crucible), or find n from a percent water. Common hydrates built in, with percent error, expected mass loss and a % water table for n = 0 to 12.
How it works
% water = n × 18.015 ÷ (M_anhydrous + n × 18.015) × 100 · from heating: n = (water lost ÷ 18.015) ÷ (residue mass ÷ M_anhydrous) · from % water: n = (%w ÷ 18.015) ÷ ((100 − %w) ÷ M_anhydrous)
A hydrated salt holds water molecules in its crystal lattice in a fixed ratio, written ·nH₂O. That water weighs n × 18.015 g/mol, and the hydrate weighs the anhydrous salt plus that water, so the water share is the water mass divided by the hydrate molar mass. The calculator works three ways. From the formula it gives the theoretical percentage and, if you enter a sample mass, how much water a heating experiment should drive off. From lab masses (before and after heating, with the empty crucible subtracted if you weighed in it) it converts the water lost and the residue to moles and divides them to get n, the hydrate formula, the measured percentage and the percent error against the expected formula. From a percent water it solves for n on a 100 g basis. Built-in salts use molar masses from the IUPAC 2021 standard atomic weights; choose Other for any salt or for a partly dried hydrate. Lab ratios are rarely exact: incomplete heating makes n too low, and spattering or decomposition of the salt makes it too high.
Worked example
Copper(II) sulfate pentahydrate, CuSO₄·5H₂O (the default): anhydrous mass 159.60 g/mol, water = 5 × 18.015 = 90.075 g/mol, hydrate = 249.675 g/mol. % water = 90.075 ÷ 249.675 × 100 = 36.077%. In the lab mode, 2.500 g of blue crystals heated to 1.600 g of white powder lost 0.900 g of water: 0.900 ÷ 18.015 = 0.04996 mol of water and 1.600 ÷ 159.60 = 0.01003 mol of CuSO₄, a ratio of 4.98, so n = 5 and the measured water is 36.000%, a 0.21% error.
Sources: Percent of water in a hydrate — LibreTexts Chemistry · Determine the formula of a hydrate — ChemTeam · Water of crystallization — Wikipedia
Frequently asked questions
What is water of crystallization?
Water held in a fixed ratio inside a crystal’s structure, such as the five waters in CuSO₄·5H₂O. Heating drives it off, often with a colour change: blue copper sulfate turns into a white powder and pink cobalt chloride turns blue.
How do I find the formula of a hydrate from an experiment?
Weigh the hydrate, heat it to constant mass and weigh the residue. Water lost ÷ 18.015 gives the moles of water; residue ÷ the anhydrous molar mass gives the moles of salt. Their ratio is n. Choose Lab masses to have the calculator do it.
Why is my n not a whole number?
Experimental error. If n is low, the sample was probably not heated long enough. If it is high, some salt may have spattered out or decomposed, which counts as extra water lost. Round to the nearest whole number unless the ratio is very close to a half.
What molar mass do I use for water?
18.015 g/mol: 2 × 1.008 for hydrogen plus 15.999 for oxygen. The calculator multiplies it by the number of water molecules.
Do I subtract the crucible mass?
Yes. If you weighed the sample in a crucible, subtract the empty crucible from both readings. Enter the empty crucible mass under More options and the calculator does it for you.
What is the percent water of Epsom salt?
MgSO₄·7H₂O is 51.17% water by mass: 7 × 18.015 = 126.105 g/mol of water in 246.465 g/mol of hydrate. Pick magnesium sulfate in the list to see it.
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This calculator is for educational purposes. Double-check important results before acting on them.