Gravimetric Stoichiometry Lesson



How to Solve Gravimetric Stoichiometric Chemistry Problems

Steps

Gather Information

  1. Read the proposed problem and take note of any reactants or products mentioned.You'll need to use this in a moment to write your reaction equation.
    • Calculate the mass ofiron (III) oxideproduced by the reaction of 500g ofironwithoxygenfrom the air.
  2. Take note of any states mentioned.This can either be solid (s), gas (g), liquid (l), or aqueous (aq). Aqueous simply means dissolved in water, and is often written in problems as "solution". Aqueous does not mean liquid! If no state is given and it is anelement, use the state on the periodic table. If it is a compound that is a reactant, you can write it as (aq).
    • Calculate the mass of iron (III) oxide produced by the reaction of 500g ofironwithoxygenfrom the air.
  3. Take note of any numbers.While dealing with Gravimetric stoichiometry, these will be either weights or moles of reactants or products.
    • Calculate the mass of iron (III) oxide produced by the reaction of500gof iron with oxygen from the air.

Write a Balanced Equation

  1. Write down any reactants on the left side of the equation and any products on the right.If the products are not given, you can predict them using other methods (article coming soon).
    • Fe(s)+ O2(g)-> Fe2O3(s).
  2. Balance both sides of the equation by writing coefficients in front of the element or compound.The total amount of individual elements comprising the reactants should equal the amount of individual elements comprising the products after the coefficients are placed. If a coefficient is one, you do not need to place a number.
    • 4Fe(s)+ 3O2(g)-> 2Fe2O3(s).
  3. Write down underneath the respective element or compound the information provided in the problem.Include proper units. Place "m = ?" under the compound or element you are solving for.

Solve the Problem

  1. Use the formula n = m / M to determine the moles of the given element or compound.n is number of moles, m is mass in grams, and M is molar mass in grams per mole. To calculate M, add up all the molar mass of the elements present in the compound found on the periodic table of elements.
    • n = m / M = 500g / 55.85g/mol = 8.95mol
  2. Use the mole ratio to get required number of moles from the given.To do this, use g * (R / G), where g is the previously calculated moles, R is the coefficient for the required mass, and G is the coefficient for the given mass.
    • g * (R / G) = 8.95mol * (2 / 4) = 4.48mol
  3. Use this new calculation to solve the required mass.Once again, use the formula n = m / M.
    • n = m / M -> 4.48mol = m / 159.70g/mol = 715g
  4. Present your final answer with correct units and significant digits/figures.

Community Q&A

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  • Question
    How do you solve for moles when only the chemical formula is given?
    wikiHow Contributor
    Community Answer
    If you know the chemical formula, you can find the molar mass by adding the molar masses of the atoms that comprise the molecule. This will give you how many moles of the substance there are per grams of that substance. Once you know this, it's easy to convert between grams to moles.
    Thanks!
Unanswered Questions
  • In gravimetric analysis how do I calculate the mass of my reactant with the unknown element in it?
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Video

  • To receive full marks, remember to always include states of matter in the balanced equation, units with every number, and significant digits in the final answer.

Warnings

  • Ensure you are keeping the mass in grams, not milligrams or kilograms.
  • Do not round or truncate answers you calculate at any time except for the end. This will often result in inaccurate answers and a loss of marks.





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Date: 08.12.2018, 15:49 / Views: 63155