Stoichiometry - Mass

Purpose

• We previously covered stoichiometry with moles. The problem with moles is that we do not directly measure moles. For many chemical reactions, we measure quantities in grams. We, therefore, need to be able to do our stoichiometry with masses in grams.

Remember – the Balanced Equation Gives the Mole Ratio

• The keys to stoichoimetry (even with mass) are the coefficients in the balanced chemical equation:

2 H2 + O2 2 H2O

• The coefficients in the balanced equations can be thought of in terms of moles but not mass:

• To use the mole ratio you must first convert the given mass to moles!

2 H2 + O2 → 2 H2O

2 mole 1 mole 2 mole

Example

• What mass of O2 will react with 5.00 grams of H2?

2 H2 + O2 2 H2O

– From the balanced equation, 2 moles of H2 react with 1 mole of O2.– Since we are given a mass, we must first convert it to moles:

– We can now set-up the conversion from mole of H2 to mole of O2

– So, 1.25 moles of O2 will react; however, we are asked for what mass.

Convert the moles to mass:

Misconception alert

• The coefficients can only be used when you are in moles. Do not use them with grams!

• The coefficients are only used when going from moles of A to moles of B. They are not used when converting mass to moles or moles to mass. Do not use them with the molar masses!

Example - all in one calculation

• What mass of O2 will react with 5.00 grams of H2?

2 H2 + O2 2 H2O– We can put all of the calculations together in one dimensional analysis

equation:

– When doing it in a single step, make sure you understand each step in the process, and make sure to only use the coefficients in the mole ratio.

Pause and practice

3 Fe(NO3)2 + 2 Na3PO4 Fe3(PO4)2 + 6 NaNO3

• How many grams of iron(II) nitrate will react with 0.24 grams of sodium phosphate?

• How many grams of sodium nitrate will be produced from the complete reaction of 3.23 grams of iron(II) nitrate.

Pause and Practice Answers

3 Fe(NO3)2 + 2 Na3PO4 Fe3(PO4)2 + 6 NaNO3

• How many grams of iron(II) nitrate will react with 0.24 grams of sodium phosphate?

• How many grams of sodium nitrate will be produced from the complete reaction of 3.23 grams of iron(II) nitrate.

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