Mole Ratios

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How to Write Mole Ratios

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Key Questions

  • Answer:

    The mole ratio is the ratio of moles of one substance to the moles of another substance in a balanced equation.

    Explanation:

    2 H2(g) + O2(g) → 2 H2O(g)

    The mole ratio between O2 and H2O is 1:2. For every 1 mole of O2 used, 2 moles of H2O are formed.

    The mole ratio between H2 and H2O is 1:1.

    For every two moles of H2 used, 2 moles of H2O is formed.

  • Mole ratios are used as conversion factors between products and reactants in stoichiometry calculations.

    For example, in the reaction

    2H₂(g) + O₂(g) → 2H₂O(g)

    The mole ratio between O₂ and H₂O is #(1 mol O₂)/(2 mol H₂O)#.

    The mole ratio between H₂ and H₂O is #(2 mol H₂)/(2 mol H₂O)#.

    Example:

    How many moles of O₂ are required to form 5.00 moles of H₂O?

    Solution:

    5.00 mol H₂O × #(1 mol O₂)/(2 mol H₂O)# = 2.50 mol O₂.

    If the question had been stated in terms of grams, you would have had to convert grams of H₂O to moles of H₂O, then moles of H₂O to moles of O₂ (as above), and finally moles of O₂ to grams of O₂.

  • Answer:

    To get the experimental molar ratio, you divide the moles of each reactant that you actually used in the experiment by each other.

    Explanation:

    EXAMPLE 1

    Consider the reaction: #"2Al" + "3I"_2 → "2AlI"_3#

    What is the experimental molar ratio of #"Al"# to #"I"_2# if 1.20 g #"Al"# reacts with 2.40 g #"I"_2#?

    Solution

    Step 1: Convert all masses into moles.

    #1.20 cancel("g Al") × "1 mol Al"/(26.98 cancel("g Al")) = "0.044 48 mol Al"#

    #2.40 cancel("g I₂") × ("1 mol I"_2)/(253.8 cancel("g I₂")) = "0.009 456 mol I"_2#

    Step 2: Calculate the molar ratios

    To calculate the molar ratios, you put the moles of one reactant over the moles of the other reactant.

    This gives you a molar ratio of #"Al"# to #"I"_2# of #0.04448/0.009456#

    Usually, you divide each number in the fraction by the smaller number of moles. This gives a ratio in which no number is less than 1.

    The experimental molar ratio of #"Al"# to #"I"_2# is then #0.04448/0.009456 = 4.70/1# (3 significant figures)

    The experimental molar ratio of #"I"_2# to #"Al"# is #1/4.70#

    Note: It is not incorrect to divide by the larger number and express the above ratios as 1:0.213 and 0.213:1, respectively. It is just a matter of preference.

    EXAMPLE 2

    A student reacted 10.2 g of barium chloride with excess silver nitrate, according to the equation

    #"BaCl"_2("aq") + "2AgNO"_3("aq") → "2AgCl(s)" + "Ba(NO"_3)_2("aq")#

    She isolated 14.5 g of silver chloride. What was her experimental molar ratio of #"AgCl"# to #"BaCl"_2#?

    Solution

    Step 1: Convert all masses into moles

    #10.2 cancel("g BaCl₂") × ("1 mol BaCl"_2)/(208.2 cancel("g BaCl₂")) = "0.048 99 mol BaCl"_2#

    #14.5 cancel("g AgCl") × "1 mol AgCl"/(143.3 cancel("g AgCl")) = "0.1012 mol AgCl"#

    Step 2: Calculate the molar ratios

    The experimental molar ratio of #"AgCl"# to #"BaCl"_2# is #0.1012/0.04899 = 2.07/1#

    Here is a video example:


    video from: Noel Pauller

Questions

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