Guess how many pennies are in the container. In a similar way, chemists use the relationships between the mole and quantities such as mass, volume, and. We can add that conversion factor as another step in a calculation to make a mole-mass calculation, where we start with a given number of moles of a substance. MOLE-MASS AND MOLE-VOLUME. RELATIONSHIPS. Section Review. Objectives. • Convert the mass of a substance to the number of moles of a substance.
The molar gas volume in calculations, moles, gas volumes and Avogadro's Law Avogadro's Law states that equal volumes of gases under the same conditions of temperature and pressure contain the same number of molecules.
This means equal amounts of moles of gases occupy the same volume under the same conditions of temperature and pressure. So the volumes have equal moles of separate particles molecules or individual atoms in them. Therefore one mole of any gas formula mass in gat the same temperature and pressure occupies the same volume.
Sometimes 20oC is treated as room temperature, which means an error of 1. The molar volume for s. Some handy relationships for substance Z below: This has been done experimentally in the past, but these days, molecular mass is readily done very accurately in a mass spectrometer.
In the following examples, assume you are dealing with room temperature and pressure i. Apart from solving the problems using the mole concept method a below, and reading any equations involved in a 'molar way' It is also possible to solve them without using the mole concept method b below.
You still use the molar volume itself, but you think of it as the volume occupied by the formula mass of the gas in g and never think about moles! Methods of measuring how much gas is formed volume can be compared with theoretical prediction!
You must make sure too much gas isn't produced and too fast!
A gas syringe is more accurate than collecting the gas in an inverted measuring cylinder under water shown below, but its still only accurate to the nearest cm3. You can collect any gas by this method.
That is, in step one you convert grams to moles divide by molar mass and in the third step you convert moles to grams multiply by molar mass. Alternatively, this may visually represented in a simplified manner: In this case, either the mass of a compound will be given and the volume of another is asked, or the volume of a gas will be given and the mass of another compound will be asked.
Reconsider the equation from above: Determine the volume of carbon dioxide gas that will be produced from First, it's important to understand the concept of STP, standard temperature and pressure. Standard temperature and pressure is a set of conditions Although many authors will assume STP unless otherwise specified, it is important to determine the conditions of the reaction.
If the reaction is not occuring at STP, the conversion factor given above cannot be used. The problem can still be solved, but the ideal gas law must be incorporated. Note the usage of the aforementioned conversion factor in the solution: Again, the relationship between 1 mole of gas at STP and the molar volume of Consider the reaction below: What is the volume of ammonia gas will react with Note that the first and last step in a volume-volume problem will cancel each other.
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This is because the first step, converting to liters of oxygen to moles, requires a division by In the third step, conversion of moles of ammonia to liters, requires multiplication by These two steps cancel each other and render step two mole to mole ratio the only important step.
It needs to be stressed that this only happens in a volume-volume problem. What if the question had asked to determine the mass of carbon monoxide produced from However, the first two steps of the problem remain unchanged. This is because the first step requires converting mass to moles. The second step involves a mol-mol ratio, once again pressure and temperature are immaterial. The final step involves calculating a volume of gas.
It is at this point that the ideal gas law is used. After these first two steps, the following can be determined: The variable P represents pressure, and must be in atm. The variable V is the volume, and is what we are solving for.
The variable n represents moles, and 0. The variable R is the gas law constant and has a value of 0. It is for this reason that pressure must be in atmospheres.
The temperature T must be in kelvin. First, let's make the necessary conversions for temperature and pressure. For temperature, to convert degrees Celsius to kelvin, add Solving the above proportion gives a value of 1. Problems - Volume of Liquids On occasion, a liquid reactant may be used and the mass is not given.
Instead, the volume of the liquid is given as the starting quantity.
Be careful with this as If lucky, the density of the liquid will be given in the problem. If not, then it must be found in literature. Using the density formula, the mass of the substance can be found mass equals volume multiplied by density and from there, the moles of the substance can be found.
When included on the products side, the reaction is exothermic. If included on the reactants side, the reaction in endothermic. Either way, a mole-enthalpy ratio can be generated to determine a relationship between enthalpy, mass, volume, or any other stoichiometric quantity.
Consider the exothermic reaction shown below: Consider the following question: What mass of europium will yield kJ of heat? Unlike previous stoichiometry problems that required three steps to solve, this one will only need two. This is because the step that integrates a mole-mole ratio will be replaced with a mole-enthalpy ratio.
This allows for a unit conversion moles to kilojoules and a stoichiometric ratio based on the reaction equation to be completed in one step. Problems - Limiting Reactant In the previous example, it was assumed that there was an unlimited supply of carbon monoxide to react with all of the iron. Sometimes this is not an appropriate or plausible assumption.
Sometimes two distinct masses of reactants are given, and it cannot be assumed that they will consume each other completely. Imagine trying to bake a cake. The recipe states that two eggs are needed to make a cake.
With a dozen eggs available, six cakes can be made. What if the recipe also states that a cup of sugar is necessary and only four cups of sugar are available?