Molecular weight starts with a correctly written formula
A molecular weight calculation depends on the chemical formula exactly as entered. Element symbols, subscripts, parentheses, and hydrate notation all affect the atom count. One missing subscript can change the final mass completely, so the formula should be checked before trusting the result.
The calculator adds the atomic mass contribution for each element. That contribution is the atomic mass multiplied by the number of atoms of that element in the formula.
Capitalization changes element symbols
Chemical symbols are case sensitive. Co is cobalt, while CO means carbon and oxygen. Na is sodium, while NA is not the same formula notation. Entering capitalization correctly is not cosmetic; it changes which elements are counted.
When copying from a worksheet or label, preserve uppercase and lowercase letters exactly. That is the safest way to avoid interpreting the formula as a different substance.
Subscripts multiply the element before them
A subscript shows how many atoms of the preceding element are present. H2O has two hydrogen atoms and one oxygen atom. C6H12O6 has six carbon atoms, twelve hydrogen atoms, and six oxygen atoms. If no subscript is written, the count is one.
Subscripts should not be confused with coefficients in a chemical equation. A coefficient multiplies the whole molecule in a reaction, while molecular weight is usually calculated for one formula unit.
Parentheses multiply grouped atoms
Parentheses collect a group of atoms and a subscript outside the parentheses multiplies the whole group. In Ca(OH)2, the OH group appears twice, so there is one calcium, two oxygen atoms, and two hydrogen atoms.
Nested or complex groups should be expanded carefully. The calculator can help, but the formula entry still has to match the intended compound.
Hydrates add water units to the mass
Some formulas include water of hydration, often written with a dot. That dot means the water molecules are included in the formula mass. A hydrate formula is heavier than the anhydrous compound because the water portion contributes atoms too.
If the interface or notation does not support a hydrate shortcut, expand the water portion according to the hydrate number before calculating.
Molecular weight supports mole conversions
Molecular weight connects grams and moles. Grams divided by molar mass gives moles, and moles multiplied by molar mass gives grams. This is often the step before a concentration or reaction calculation.
For solution concentration after moles are known, the Molarity Calculator can use moles and liters to find molarity.
Formula mass and molecular weight depend on context
For covalent molecules, molecular weight is a familiar phrase. For ionic compounds, formula mass is often the more precise classroom term because the formula represents a ratio of ions. The arithmetic is still the sum of atomic masses from the written formula.
Use the term expected by the course, lab, or reference source. The calculation method is usually the same for the entered formula.
Atomic masses are averaged values
Periodic-table atomic masses are usually weighted averages of naturally occurring isotopes. That is why many values are decimals rather than whole numbers. Isotope-specific problems may use a different exact mass when the isotope is named.
For ordinary chemistry homework, use the atomic masses from the table or calculator source specified by the class.
Charge does not usually change formula mass
An ion charge changes electron count, but ordinary formula mass calculations usually sum atomic masses from the nuclei and atoms in the formula. The charge notation itself is not treated like another atom.
If a formula includes waters, ligands, or counterions, those written pieces do contribute mass. The formula text controls what is counted.
Round according to the data source
Different periodic tables round atomic masses differently. Small differences in the final molecular weight can come from those rounding choices. Use the same table throughout a class assignment or lab report when consistency matters.
Keep enough digits for mole calculations, then round the final answer according to the significant-figure rule being used.