Chemical Equations and Stoichiometry

• The mathematical calculation of the quantitative relationship between reactants and products in a chemical reaction- which is called a Stoichiometric problem – requires the use of a process called the mole-ratio method.
• Now calculate the mole ratio by using the formula:

  mole ratio = moles of desired substance/moles of starting substance
• The Gas Law

  Measuring the Pressure of Gases

  • There many units used to measure. We shall look at only two of these: the atmosphere and the torr. The simplest unit of pressure measurement is the atmosphere, abbreviated as atm. 1 atm is the average pressure of the atmosphere at sea level. Pressure twice as great as the average pressure at sea level would equal 2 atm.
• The Gas Law

  Measuring the Pressure of Gases

  • In a barometer 1 atm will push a mercury 760 mm high into the vacuum tube. We say that the pressure is “760 torr” when the column of mercury is this high. The torr, the second unit used to measure pressure, was named in honor of TORRICELLI, who invented barometer.
• The Effect of Pressure on Gases: Boyle’s Law.

  • The volume of a gas is inversely proportional to the pressure at constant temperature. This means that, when pressure increases, the volume decreases, and vice versa. Thus, when the pressure on a quantity of gas is doubled, the volume decreases to one-half.
• If the pressure on a quantity of gas is reduced to one-half, the volume will double. Robert Boyle was the man who first discovered this relationship, and the relationship has become to be known as BOYLE’S LAW.
• Boyle’s Law is best expressed with the following equations.

  P1V1 = P2V2

  P1 = initial pressure on the gas (in atm or torr)

  V1 = initial volume of gas (in l or ml)

  P2 = new pressure on the gas (in atm or torr)

  V2 = new volume of the gas (in l or ml)
• The Effect of Pressure on Gases: Charles’ Law

  • The volume of a gas is directly proportional to the temperature at constant pressure. This means that when the temperature increases, the volume increases. This relationship between volume and temperature is known as CHARLES’ LAW.
• Charles” law is best expressed with the following equation.

  V1/T1 = V2/T2

  T1 = initial temperature of gas (K)

  V2 = initial volume of gas (in ml or L)

  T2 = new temperature of the gas (K)

  V2 = new volume of the gas (in ml or L)
• The General gas Law

  • Boyle’s Law describe the effect of changing gas on the volume of a gas when the temperature is held constant. Charles’ law describes the effect of changing temperature on a gas when the pressure is held constant. The formula describing the change in volume when both pressure and temperature change is known as General Gas Law or Combined Gas Law)