ChemTeam: Calculations involving molality, molarity, density, mass percent, mole ratio
Dilution calculations can be performed using the formula M1V1 = M2V2. Solving Dilution Problems in Solution Chemistry CLEAR & SIMPLE Translate between molarity, grams of solute in solution, and volume of .. Calculate concentrations of solutions in molarity, molality, mole fraction and percent by mass and volume. Molarity (M) is the concentration of a solution expressed as the number of moles of Instead of calculating the moles of solute and liters of solution present. Essentially, all stoichiometry problems can be broken down into three steps: 1. Again, the relationship between 1 mole of gas at STP and the molar volume of.
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.
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.
3.9 Stoichiometric Calculations: Amounts of Reactants and Products
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? Regardless of the dozen eggs, only four cakes can be made, because after consuming four cups of sugar with eight eggsthere will be no sugar remaining. At this point, no more cakes can be made.
Sugar is considered the limiting reactant in this example. How many moles of carbon are there? Given volume and molarity, it is possible to calculate mole or use moles and molarity to calculate volume.
Molarity: how to calculate the molarity formula (article) | Khan Academy
This is useful in chemical equations and dilutions. Example 7 How much 5 M stock solution is needed to prepare mL of 2 M solution? These ratios of molarity, density, and mass percent are useful in complex examples ahead. Determining Empirical Formulas An empirical formula can be determined through chemical stoichiometry by determining which elements are present in the molecule and in what ratio. The ratio of elements is determined by comparing the number of moles of each element present.
Combustion of Organic Molecules 1. What is the empirical formula of the organic molecule? The problem requires that you know that organic molecules consist of some combination of carbon, hydrogen, and oxygen elements. With that in mind, write the chemical equation out, replacing unknown numbers with variables. Do not worry about coefficients here. This will give you the number of moles from both the unknown organic molecule and the O2 so you must subtract the moles of oxygen transferred from the O2.
Moles of oxygen in CO2: With this we can use the difference of the final mass of products and initial mass of the unknown organic molecule to determine the mass of the O2 reactant. Determining Molecular Formulas To determine a molecular formula, first determine the empirical formula for the compound as shown in the section above and then determine the molecular mass experimentally.
Next, divide the molecular mass by the molar mass of the empirical formula calculated by finding the sum the total atomic masses of all the elements in the empirical formula. Multiply the subscripts of the molecular formula by this answer to get the molecular formula.
Example 9 In the example above, it was determined that the unknown molecule had an empirical formula of CH2O.
Find the molar mass of the empircal formula CH2O. Determine the molecular mass experimentally. For our compound, it is Divide the experimentally determined molecular mass by the mass of the empirical formula.