Mass Stoichiometry

Stoichiometry and Mass

Since stoichiometry uses ratios, we can only use it when we have moles or molecules of a substance! This means we have to make sure to convert to moles wherever possible:

Mg weighs 24.305 g/mol and O weighs 15.999 g/mol, therefore:

24.305 g of Mg and 31.998 g of O2 would make 2 mol of MgO.
48.61 g of Mg and excess O2 would make 4 mol of MgO.

Converting from grams to moles requires dividing by molar mass (just like we did a few weeks ago)!

We can therefore combine stoichiometry with mole conversions to find out how many grams of reactants form how many grams of product, something useful for making any sort of material with a chemical reaction. Doing this calculation is a multi-step process:

Convert from grams to moles
Use stoichiometric ratios
Convert from moles back to grams (this time though it should be grams of the molecule you were looking for)

Example Problem: How many moles of oxygen gas would be made from 72g of H2O decomposing?

The molar mass of H2O is 18 g/mol (from adding the masses of 1 oxygen and 2 hydrogen on the periodic table).
72g / 18g/mol = 4 moles of H2O are decomposing
For every 2 H2Os we get 1 O2, so 2 moles of O2 are made.
The molar mass of O2 is 32 g/mol
Molar Mass x Moles = Mass (32 x 2 = 64), so 64g of oxygen gas were made!

Stoichiometry and Error

When doing a physical reaction you'll often get error compared to your stoichiometric calculations, similar to all other processes we've covered thus far. Percent Error and Percent Yield for stoichiometry problems are as follows:

Percent Error = (Difference between measured and calculated values) / (Calculated value) * 100%

Percent Yield = (Actual Value) / (Calculated Value) * 100%

Toxicology (PSA)

Many times when dealing with toxic chemicals in the body the dosage, location of entry, phase of matter, the size and weight of the patient, and more determine how dangerous a chemical is to someone.

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