Chemistry · Some Basic Concepts Of Chemistry · NEET
The way you calculate them is identical: you add up the atomic masses of all atoms shown in the formula. The number you get is the same style of number. The ONLY difference is the kind of substance you use the word for. If the substance is made of true molecules (separate particles like H2O or CH4), we call it molecular mass. If the substance is an ionic solid that has no separate molecules (like NaCl), we call it formula mass. So they are not different math, they are different names for different substance types.
Use formula mass whenever the compound is ionic and does NOT exist as single, separate molecules. NCERT says sodium chloride does not contain discrete molecules; instead sodium ions and chloride ions sit in a repeating 3D grid. Because there is no single NaCl molecule floating around, we cannot honestly call it a molecular mass. So we use the word formula mass. This is true for most salts: NaCl, KCl, CaO, MgCl2, Na2CO3, and so on.
In solid NaCl there is no unit that is one sodium joined to one chlorine as a private molecule. Every Na+ ion is surrounded by many Cl- ions, and every Cl- ion is surrounded by many Na+ ions. The formula NaCl only tells us the ratio (1 sodium : 1 chlorine), not a real molecule. Since there is no molecule, the term molecular mass does not fit, so we use formula mass = 23.0 + 35.5 = 58.5 u.
No. Water (H2O), methane (CH4), glucose (C6H12O6) and other covalent compounds DO exist as real separate molecules. So for these we use the term molecular mass, not formula mass. Molecular mass of water = 2(1.008) + 16.00 = 18.02 u. Molecular mass of glucose = 6(12.011) + 12(1.008) + 6(16.00) = 180.162 u.
Step 1: write the formula, NaCl. Step 2: list the atomic masses: Na = 23.0 u, Cl = 35.5 u. Step 3: multiply each by how many atoms of it are present (one Na, one Cl), then add. Formula mass of NaCl = 23.0 + 35.5 = 58.5 u. For MgCl2 it would be 24.3 + 2(35.5) = 95.3 u, because there are two chlorine atoms.
The calculated number is correct either way, so a numerical answer is usually still right. But NEET often tests the CONCEPT directly: it may ask why NaCl uses formula mass, or use the words formula unit. Using the correct term shows you understand that ionic solids have no molecules. This is a favourite trap, so learn the reason, not just the number.
Among the following, choose the ones with an equal number of atoms. A. 212 g of Na2CO3 (molar mass = 106 g) B. 248 g of Na2O (molar mass = 62 g) C. 240 g of NaOH (molar mass = 40 g) D. 12 g of H2 (molar mass = 2 g) E. 220 g of CO2 (molar mass = 44 g)
The number of hydrogen atoms present in 5.4 g of urea is (molar mass of urea = 60 g/mol; NA = 6.022x10^23 /mol)
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Both are the sum of atomic masses in a formula, but molecular mass is used for substances made of real molecules (H2O, CH4), while formula mass is used for ionic solids with no molecules (NaCl).
Formula mass of NaCl = atomic mass of Na + atomic mass of Cl = 23.0 u + 35.5 u = 58.5 u.
Molar mass is the mass of one mole in grams. Its number is equal to the molecular mass or formula mass in u. So molar mass of water is 18.02 g/mol and molar mass of NaCl is 58.5 g/mol.
No. CaO and MgCl2 are ionic solids, so the correct term is formula mass. Molecular mass is only for covalent molecules like CO2 or H2O.
NEET asks conceptual questions about why NaCl uses formula mass, and uses the phrase formula unit in atom-counting problems. Knowing which term fits which substance avoids easy trap marks.
