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The reaction is a decomposition…
… because one substance is forming two other substances.
It is also a <em>reduction-oxidation reaction</em> because the <em>oxidation number of N decreases</em> from +5 to +3 and the <em>oxidation number of O increases</em> from -2 to 0.
2NO + O₂ = 2NO₂
V(O₂)=10.5 L
V(NO₂)=2V(O₂)
V(NO₂)=2*10.5=21.0 L
21.0 liters of NO2 are produced
The Ideal Gas Law states that pressure (P) × volume (V) is equal to the # of moles (n) of the gas × a constant (R) × temperature (T), such that the equation is:
PV = nRT
At standard temp and pressure (STP), the T is 0°C or 273.15K, the P is 1 atm or 760 torr, and the R constant is 0.0821. Therefore the equation, solved for V becomes: V = nRT/P, or V = n(0.0821)(273)/1, so that it reduces to V = 22.4 Liters, when n = 1 mole.
So the V of any gas at STP is 22.4 L / mole
Matter is the correct word.