Heat a hydrate until the water is gone, work out what percent of it was water, and match it to one of four candidates. Needs a formal report.
Some ionic compounds hold water molecules inside their crystals. Those are hydrates, and the textbook covers them in section 2.7.
The name is the name of the anhydrous compound, meaning the one without water, plus a Greek prefix for the number of water molecules and the word hydrate. Copper(II) sulfate with five waters is copper(II) sulfate pentahydrate. Washing soda, sodium carbonate with ten waters, is sodium carbonate decahydrate.
The formula is written with a centered dot, the number of waters, and the formula for water. Heating drives the water off, and the reaction runs both ways:
CuSO₄ · 5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(l)
Anhydrous cobalt(II) chloride is blue and the hexahydrate is pink, which is why it turns up in humidity indicators.
Prefixes: mono 1, di 2, tri 3, tetra 4, penta 5, hexa 6, hepta 7, octa 8, nona 9, deca 10.
percent water = (mass of water lost ÷ mass of hydrate) × 100
The four candidates are CuSO₄ · 5H₂O, Na₂CO₃ · H₂O, MgSO₄ · 7H₂O and NiCl₂ · 6H₂O.
Bunsen burner, ring stand, clay triangle, and crucible with lid. Assume the crucibles are clean and dry.
Watch out: never touch the crucible or lid without gloves, always let it cool before weighing, and use the same balance every time.
Type a formula or add elements one at a time. The atomic masses are read off the periodic table built into this site, so you never copy one across. A dot separates the salt from its water, and when there is one you also get the mass of each part and the percent water.
Nothing added yet. Tap one of the hydrates below to load it, or start typing a formula.
These match the posted pre-lab answer key: sulfur 32.07, hydrogen 1.008. Do not round the water before you multiply it. The key writes water as 18.02 in its table but multiplies with 18.016, so six waters come to 108.10 and nickel(II) chloride hexahydrate to 237.69. Rounding first gives 6 × 18.02 = 108.12 and 237.71, which the key marks wrong. The calculator below keeps the full value all the way through and only rounds at the end, which is the order the key uses. Sulfur is the one value that differs from the element articles elsewhere on this site, which carry the current IUPAC 32.06.
For each one, work out the formula weight of the hydrate and the percent of that mass which is water. The first four are what your unknown could be. The last two are on the pre-lab as well, but they are not candidates, so they never appear in the identification menu below. Nothing here is calculated for you. Check marks each box: it has to be the right number, correctly rounded, with at least four sig figs on a formula weight and three on a percentage. It never shows you the value.
One water molecule is 18.016 g/mol, which the key writes as 18.02 but multiplies unrounded. The rest you look up: a formula weight is the sum of the atomic weights of every atom in the formula, counting each water as H₂O. These six agree with the posted answer key to the last digit.
The rows follow the data table in the lab notes. The blue boxes are the ones you calculate. Heat, cool and weigh until two masses in a row agree within 0.01 g, then use that last mass. Check is strict about sig figs: a subtraction keeps decimal places, a division keeps significant figures, and percent error follows the same chain.
The first thirteen come from the lab notes: what a hydrate is, and how the heating is actually done. The rest are the pre-lab itself, question for question, with the same compounds and the same numbers, ending with identifying the unknown from its percent water. Each question says only whether your answer is right; after a miss you can show it.
Lab 4 is the one that needs a full formal report, due at the start of Lab 5: typed, third person, with a title page, introduction, procedure, calculations with an example, tables with units, and a discussion that answers the objectives you set out in the introduction.
Links The checklist on the Labs page Lab 3: density Unit Tracks: conversions and sig figs