Working Out The Molar Weight Of Alum
Alum almost always means potassium aluminum sulfate dodecahydrate, KAl(SO4)2·12H2O. The dot-and-digits notation isn't decoration. Those twelve water molecules are part of the crystal lattice and they carry real mass. Leave them out and your stoichiometry will be wrong by about 45 percent. Here is how the number actually comes together, not just what the final figure is. K: 39.10 g/mol
Al: 26.98 g/mol
S: 32.07 × 2 = 64.14 g/mol
O (in sulfate): 16.00 × 8 = 128.00 g/mol
H2O (12 molecules): 18.015 × 12 = 216.18 g/mol
Add those up and you get 474.39 g/mol. Round to 474.4 g/mol for routine bench work. If you are doing analytical weighing, keep the full decimal and round at the end. I learned that the hard way once when a student used 474 g/mol for a gravimetric purity check and the result came out 0.3 percent low. The difference looked noise-level until I traced it back to the rounding. The anhydrous salt, KAl(SO4)2, sits at 258.21 g/mol. Some protocols call for the anhydrous form. More call for the dodecahydrate. Check your source before you weigh anything. Mixing the two up is one of the most common errors I see in student labs, and it usually doesn't show up until the titration curve looks wrong.
What Happens In Practice
Weighing alum sounds straightforward until you open a jar that has been sitting on the shelf for six months. The crystals look fine. They aren't. Efflorescence is real. The outer layer loses water to the air and the effective molar mass of whatever you scoop drops below 474.4 g/mol without you knowing it. You dissolve it, calculate concentrations based on the labeled mass, and your solution is slightly weaker than you think. It isn't catastrophic for a home brewing or water clarification project, but it matters if you are preparing a primary standard or running a kinetics experiment. The fix isn't fancy. Store alum in a tightly sealed container with a desiccant packet. Before using old stock, dry a small batch at 110°C for two hours, cool in a desiccator, and recheck the mass. If you need the dodecahydrate form for a protocol, just know that heating it past roughly 92°C begins driving off water. Keep the drying step short and don't exceed 120°C unless you specifically want anhydrous alum. Another thing nobody warns you about: the molar mass changes if you are working with a different alum. "Alum" is a class of compounds. Ammonium alum, NH4Al(SO4)2·12H2O, has a molar mass of about 453.2 g/mol. Chromium alum, KCr(SO4)2·12H2O, is roughly 499.4 g/mol. Sodium alum exists too but behaves differently. If your protocol just says "alum" without specifying, ask. I had a colleague prepare a mordant solution using the ammonium version at the potassium mass and wonder why his dye uptake was inconsistent. Took him a week to catch it.
Get the Full Details
Quick Reference Values
Potassium aluminum sulfate dodecahydrate: 474.39 g/mol
Potassium aluminum sulfate anhydrous: 258.21 g/mol
Ammonium aluminum sulfate dodecahydrate: 453.20 g/mol For most general purposes, including water purification dosing, pickling alum baths, or school chemistry labs, 474.4 g/mol is the number you need. Write it down. Keep it on your bench note sheet. It saves you from re-deriving it every time.
A Few Things To Watch
Pure alum solutions are stable for months if stored at room temperature in sealed containers. Light and temperature swings don't degrade them noticeably. The main degradation route is physical water loss from the crystal, not chemical breakdown in solution. That said, if you are making up a stock solution and the concentration seems to drift over weeks, check the jar seal. Evaporation concentrates the solution. It isn't the alum decomposing. If you need high precision, use an analytical balance and account for buoyancy. Air displacement can shift a 10 g weigh by a few milligrams depending on humidity and altitude. It is easy to ignore at first. It piles up fast. One more thing. The 12 waters of hydration are structural. You cannot reliably rehydrate partially effloresced crystals by simply exposing them to humid air. The water returns unevenly and you end up with a mix of hydrated and partially dehydrated phases. Better to start fresh or dry and rehydrate under controlled conditions if the protocol demands exact stoichiometry.
If you want a downloadable reference card with these values and a one-page calculation sheet, many university chemistry departments post printable versions of standard molar mass tables. Search for "analytical chemistry molar mass reference pdf" and pull one from a .edu domain. The numbers are the same everywhere. The formatting and extra compounds included vary by school. Save one and tape it near the balance.
