MASS-VOLUME STOICHIOMETRY

Not all Stoichiometry problems are mass-mass!• Sometime a reactant or product in a stoichiometry problem will be an

element/compound that exists in gas form.• Since gases are unlike solids and liquids in the fact they do not have a

defined volume, determining the amount of a gas reactant or product involved in a stoichiometric reaction is a bit different.• Fortunately, gases, like all substance, obey certain laws of physics.• The one we will use states that “under standard temperature and

pressure, one mole of a gas will occupy a volume of 22.4 liters (l)”. • This is referred to as the MOLAR VOLUME OF A GAS.

Standard Temperature and Pressure

• Standard Temperature and Pressure, or STP for short, are the condition under which gas volumes are measured.• Standard Temperature is 20.0 oC ( 68.0 oF ).• Standard Temperature is 1.00 Atomsphere (atm). This is the amount

of force that the atmosphere exerts on every square inch of an object at sea level. • Pressure is usually measured in units call mmHg. 1 atm = 760 mmHg

pressure.

Sample Problem• In the laboratory, small quantities of hydrogen gas can be generated

by reacting mossy Zinc metal (mossy means it’s tarnished!) with a strong solution of Hydrochloric Acid, HCl, and the resulting Hydrogen gas collected.• What volume, at STP, of Hydrogen gas (H2) would be produced if 75.5

grams of Zinc metal (Zn) react with an excess of Hydrochloric Acid (HCl) according to the reaction:

• Zn (s) + HCl (l) ZnCl2 (s) + H2 (g) [ an can also indicate the formation of a gas in a reaction]

• (s) = solid (l) = liquid (g or ) = gas

Other than the fact the problem asks for volume rather than mass of H2, the problem is worked just like any other stoichiometry problem!

•Zn (s) + HCl (l) ZnCl2 (s) + H2 (g) 75.0 g xs ? l

Practice Problem

• In metabolic respiration, cells break down Glucose to release energy, and, in that process, produce Water and Carbon Dioxide as by products according to the equation:

• C6H12O6 + O2 ENERGY + H20 + CO2

• What volume of Carbon Dioxide can be formed at STP from 275.0 grams of Glucose?