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Which of the following correctly represents the unit mass of a molecule of water (H тВВ O)?
18 u
10 u
16 u
20 u
18 u
The molecular mass of water (H2тАЛO) is calculated by summing the atomic masses of its constituent elements. Since hydrogen has an atomic mass of 1┬аu and oxygen has an atomic mass of 16┬аu, the total mass is 2(1┬аu)+16┬аu=18┬аu.
The molecular mass of water (H2тАЛO) is calculated by summing the atomic masses of its constituent elements. Since hydrogen has an atomic mass of 1┬аu and oxygen has an atomic mass of 16┬аu, the total mass is 2(1┬аu)+16┬аu=18┬аu.
Think of calculating the weight of a water molecule like calculating the total weight of a fruit basket: two apples (hydrogen atoms at 1┬аkg each) plus one watermelon (oxygen atom at 16┬аkg) gives a total of 18┬аkg.
Molecular┬аMass=тИС(niтАЛ├ЧAtomicMassiтАЛ) тАФ where niтАЛ is the number of atoms of element i.
Mass(H2тАЛO)=2├ЧMass(H)+1├ЧMass(O) тАФ specific calculation for water.
The principle of additivity of atomic masses applies here. Each molecule of water consists of two atoms of Hydrogen (H) and one atom of Oxygen (O). By referencing the periodic table, we use the average atomic masses (rounded to the nearest integer) and multiply them by their respective stoichiometric subscripts in the chemical formula.
Atomic mass of Hydrogen (H) тЙИ1┬аu
Atomic mass of Oxygen (O) тЙИ16┬аu
Unit 'u' stands for Unified Atomic Mass Unit, which is 1/12th the mass of a Carbon-12 atom.
Provides a standardized way to measure molecular mass across chemical compounds.
Calculation of molar mass for stoichiometry.
Determination of gas density and vapor density.
The value 18 u is the molecular mass; the molar mass is numerically equal, being 18 g/mol.
Option B (10 u) is incorrect as it underestimates the contribution of oxygen.
Option C (16 u) is incorrect as it ignores the weight of the two hydrogen atoms.
Option D (20 u) is incorrect as it suggests an extra mass contribution not present in standard water molecules.
A is correct тАФ The molecular mass of H2тАЛO is determined by adding 2├Ч1┬аu (Hydrogen) and 16┬аu (Oxygen), resulting in 18┬аu.
Always memorize the first 20 elements' atomic masses as they are frequently used in competitive exams for stoichiometry calculations.
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