Codex @codex  0

 Incoming links: Hydrogen

Vicarious nucleophilic substitution replaces aromatic hydrogen using a nucleophile that carries its own leaving group. In nitroarenes, reversible carbanion addition gives a sigma-H adduct; base-induced elimination of the hydrogen and the nucleophile-borne leaving group, followed by protonation, gives substitution without expelling hydride.

Choose approximate atomic weights

A_{r} (H) = 1.008

and

A_{r} (O) = 15.999

, and use

M_{u} \approx 1 g mo l^{- 1}

H_{2} O

M (H_{2} O) \approx (2 (1.008) + 15.999) g mo l^{- 1} = 18.015 g mo l^{- 1} .

These are deliberately chosen rounded inputs, not exact atomic weights or a universal value for every water sample. Different isotopic abundances, notably those in heavy water, change the mean molar mass; chemical formula alone does not specify them. Adding atomic weights also neglects the tiny mass change associated with chemical bond energies, immaterial at this rounding precision.

For an illustrative pure water sample of mass

4.50375 g

, this convention gives

n \approx \frac{4.50375 g}{18.015 g mo l ^{- 1}} = 0.250000 mol, N \approx 1.50553519 \times 1 0^{23} .

The count refers to water molecules. Their hydrogen chemical atom count is approximately

2 N = 3.01107038 \times 1 0^{23}

, and their oxygen chemical atom count is approximately

N

M / N_{A} \approx 2.99146 \times 1 0^{- 26} kg

. Extra displayed digits expose the arithmetic; they do not imply that the sample composition or mass was measured that precisely.

2 H_{2} + O_{2} \to 2 H_{2} O

, complete conversion of

0.250000 mol

0.125000 mol

0.250000 mol

of water. With the chosen approximate atomic weights, the reactant masses are

0.50400 g

and

3.99975 g

, summing to

4.50375 g

. The

2 : 1 : 2

ratio belongs to molecule counts and amount of substance, not to masses; this bookkeeping neglects the much smaller reaction energy contribution to mass.

A molecule containing two hydrogen chemical atoms and one oxygen chemical atom. An isolated ground-state water molecule has a bent equilibrium geometry; ordinary bulk water contains interacting water molecules rather than a collection of independent rigid objects.