Volume Relationships in Chemical Reactions
Avogadro's law is an experimental gas law relating the volume of a gas to the amount of For a given mass of an ideal gas, the volume and amount (moles) of the gas The equation shows that, as the number of moles of gas increases, the volume of . Boyle's law – Relationship between pressure and volume in a gas at. If an industrial plant must produce a certain number of tons of sulfuric acid per week, . We just used the balanced chemical equation to calculate the mass of product .. As you learned in, density is the mass per unit volume of a substance . Mass and Gaseous Volume relationships in Chemical Reactions. As we already know, chemicals come in a variety of forms and states. We use the moles.
Answer the question - convert the moles of the desired compound to the appropriate quantity i. When unsure of what the question is asking, look for clues in the wording. If a phrase such as "find the number of grams" is used, the unit grams indicates that the mass should be found.
Consider the following reaction and problem: Determine the mass of iron that is produced from This problem is often referred to as a mass-mass problem since you are given the mass of a compound in the problem and asked to find the mass of another compound. The three step method described above can be applied in the setup shown below: In the first conversion factor above, the molar mass of Fe2O3 is determined since it is needed to convert to moles step one.
Mass Relationships in Chemical Equations
When given a mass as in this problemdividing by molar mass always converts to moles. In the second conversion factor step twoa ratio of Fe to Fe2O3 is determined from the coefficients in the reaction. It is worth noting that this is the only time the coefficients of the reaction are used when solving a stoichiometry problem. Finally, the molar mass of Fe is used to convert moles of Fe to grams of Fe step three.
Volume Relationships in Chemical Reactions
It's worth noting that in a mass-mass problem, the first and third steps are opposites of each other. 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.
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.
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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.
It would seem that a water molecule, which contains at least one oxygen atom, should weigh more than a single oxygen particle. These are serious objections to the idea that equal volumes of gas contain equal numbers of particles. Our postulate appears to have contradicted common sense and experimental observation. However, the simple ratios of the Law of Combining Volumes are also equally compelling.
Why should volumes react in simple whole number ratios if they do not represent equal numbers of particles? Consider the opposite viewpoint: Now when we combine particles in simple whole number ratios to form molecules, the volumes of gases required would produce decidedly non-whole number ratios. The Law of Combining Volumes should not be contradicted lightly. There is only one logical way out. We will accept our deduction from the Law of Combining Volumes that equal volumes of gas contain equal numbers of particles, a conclusion known as Avogadro's Hypothesis.
How do we account for the fact that 1 liter of hydrogen plus 1 liter of chlorine yields 2 liters of hydrogen chloride? There is only one way for a single hydrogen particle to produce 2 identical hydrogen chloride molecules: In fact, each hydrogen particle or molecule must contain an even number of hydrogen atoms. Similarly, a chlorine molecule must contain an even number of chlorine atoms.
We will assume the simplest possibility and see if that produces any contradictions. This is a wonderful result, for it correctly accounts for the Law of Combining Volumes and eliminates our concerns about creating new atoms. Most importantly, we now know the molecular formula of hydrogen chloride.
We have, in effect, found a way of "counting" the atoms in the reaction by measuring the volume of gases which react. This method works to tell us the molecular formula of many compounds. Recall that the mass ratio in water is 8: Since there are two hydrogen atoms for every oxygen atom in water, then the mass ratio requires that a single oxygen atom weigh 16 times the mass of a hydrogen atom.
To determine a mass scale for atoms, we simply need to choose a standard. For example, for our purposes here, we will say that a hydrogen atom has a mass of 1 on the atomic mass scale. Then an oxygen atom has a mass of 16 on this scale. Our conclusions account for the apparent problems with the masses of reacting gases, specifically, that oxygen gas weighs more than water vapor.
This seemed to be nonsensical: However, this is now simply understood: