Making a solution from a solid is a different job from diluting a liquid, and it needs a different calculation. Dilution starts with a strong liquid and asks how much of it to take. Preparation from a solid starts with an empty flask and asks how much powder to weigh. The answer comes from three numbers: the molarity you want, the final volume you want, and the molecular weight of the solid. Multiply them, with the volume in litres, and you get grams.
The formula is short: mass equals molarity times volume in litres times molecular weight. Molarity is moles per litre, so molarity times litres gives moles, and moles times grams per mole gives grams. The units cancel neatly, which is a good check: if your units do not cancel to grams, one input is in the wrong unit. The most common slip is entering the volume in millilitres without converting, which makes the answer a thousand times too big.
Here is a worked example. You want one litre of a 1 molar salt solution, and salt, sodium chloride, has a molecular weight of 58.44 grams per mole. The maths is 1 times 1 times 58.44, which is 58.44 grams. Weigh that much salt, dissolve it in some water, and then add water up to exactly one litre. A smaller example: 250 millilitres of 0.5 molar glucose, molecular weight 180.156. The maths is 0.5 times 0.25 times 180.156, which is 22.52 grams.
The order of steps matters as much as the number. Weigh the solid first, dissolve it in part of the solvent, and only then fill up to the final volume mark. If you weigh the solid into a full litre of water, the solid takes up space and the final volume is more than a litre, so the concentration is lower than planned. A volumetric flask exists for exactly this reason: it lets you fill to a precise mark after dissolving, and that mark is the volume in the calculation.
Molecular weight is the input students get wrong most often, because it must match the exact solid in the bottle. Some solids carry water in their crystals, and that water counts in the weight. Copper sulfate pentahydrate is not the same as anhydrous copper sulfate, and using the wrong weight gives the wrong molarity. Read the bottle label, check the formula including any waters of crystallisation, and use a molecular weight calculator to add the atoms up before you use this tool.
Purity matters too, and it is the quiet error in many school results. A bottle labelled 98 percent pure contains 2 percent of something else, so weighing the calculated mass gives slightly less of the real compound than planned. For school practice, ignore purity unless your teacher says otherwise. For any serious work, the correction is to divide the calculated mass by the purity as a decimal: 10 grams at 98 percent purity becomes 10 divided by 0.98, which is 10.20 grams.
Use this calculator as a practice aid for study and homework, not as lab procedure. It is an educational estimator, and real solution preparation follows a written protocol, a supervisor, and the safety data sheet for the solid involved. Some solids are corrosive, toxic, or dusty, and weighing them needs protective equipment and training that no web tool can give. Check every result against your protocol before any real step, and if a number looks surprising, redo it by hand first.
Stock solutions save weighing tiny masses. If you need 10 millilitres of 0.01 molar solution, the mass is 0.01 times 0.010 litres times the molecular weight, which for salt is 0.0058 grams, far too small for a school balance. The fix is to make a stronger stock, say 0.1 molar in 100 millilitres, which needs a weighable 0.5844 grams, and then dilute a measured part of it to the target. That two-step route, prepare then dilute, is standard practice, and it is why preparation and dilution tools sit side by side: preparation answers how much solid to weigh for the stock, and dilution answers how much stock to take for the final volume.