AP Chemistry

Explore

Pages

Notes

3. Stoichiometry

Isotopes

isotopes: two atoms have the same number of protons but different numbers of neutrons

example:

Hydrogen-1: 1 proton, 0 neutrons

Hydrogen-2: 1 proton, 1 neutron

Hydrogen-3: 1 proton, 2 neutrons

mass spectrometer: machine that helps you figure out what percent of each isotope you have

⁠

⁠

relative abundance: bar graph showing ratios

finding element from relative abundance graph: find element close to highest bar

mole (mol): 6.022 ⨉ 10²³

based on number of atoms in 12g of carbon

average atomic mass based on mass in 1 mole of element

calculating atomic mass of compounds: add atomic masses of components

chemical formulas tell mole ratios

Formulas

empirical formula: a compound ratio that cannot be reduced

An unknown compound contains 38.67% carbon, 16.22% hydrogen, and 45.11% nitrogen. What is the empirical formula?

change percentages into grams (100g)

38.67 g C

16.22 g H

45.11 g N

convert from grams to moles (dimensional analysis)

3.220 mol C

16.09 mol H

3.220 mol N

divide everything by the smallest number of moles

3.220/3.220 = 1 part C

16.09/3.220 = 5 parts H

3.220/3.220 = 1 part N

rewrite parts as empirical formula

CH₅N

molecular formula: a compound ratio that can be reduced

The molecular mass of CH₅N is 62.12 g/mol. What is the molecular formula?

take empirical formula and find molar mass

1 mol (12.011 g/mol) + 5 mol (1.009 g/mol) + 1 mol (14.007 g/mol) = 31.06 g

divide molecular mass by empirical mass

62.12 g / 31.06 g = 2

rewrite empirical formula as molecular formula

2 (CH₅N) = C₂H₁₀N₂

Balancing equations

⁠

reactants (left side)

yields (arrow)

products (right side)

state of matter

(g): gas

(l): liquid

(s): solid

(aq): aqueous

C₄H₁₀ + O₂ → CO₂ + H₂O

2 C₄H₁₀ + 13 O₂ → 8 CO₂ + 10 H₂O

Stoichiometry

math in chemistry

example: What mass of oxygen will react with 96.1 g of C₃H₈?

C₃H₈ (g) + 5O₂ (g) → 3 CO₂ (g) + 4H₂O (g)

change grams to moles

⁠

use a ratio to change to unit you are looking for

⁠

change from moles to grams

⁠

limiting reactant: the reactant which runs out first in a reaction

example: 2 slices of bread + 1 piece of meat + 1 piece of cheese = 1 sandwich

7 pieces of bread + 10 pieces of meat + excess cheese = 3 sandwiches

limiting reactant: bread

example: 2 NH₃(g) + 3 CuO (s) → N₂ (g) + 3 Cu (s) + 3 H₂O (g); 81.1 g NH₃, 90.4 g C₄O

which is the limiting reactant?

⁠

CuO is the limiting reactant

how many grams of N₂ are formed?

⁠

percent yield

⁠

theoretical yield: answer from the problem

actual yield: what happened in the experiment

example (from limiting reactant problem): actual yield was 6.63 g N₂

⁠

Want to print your doc?
This is not the way.

Try clicking the ⋯ next to your doc name or using a keyboard shortcut (

CtrlP

) instead.