Test 9- Stoichiometry Flashcards
Atomic mass=
average mass of all isotopes of the atom
Isotopes
same number of protons but different number of neutrons
Mass of an atom is measured in
a unit called a.m.u (atomic mass unit)
amu
1/12 the mass of a carbon-12 atom, which is the relative unit
Avogadro’s number
6.02 X 10^23
1 mole=
6.02 X 10^23 particles (also a relative unit)
Particles
formula unit, atom, or molecule
Avogadro’s number=
the mole
6.02 X 10^23 is equal to
the mass in grams on the periodic table
- Ex. 1 mole Al=6.02 X 10^23, 1 mole Fe=6.02 X 10^23, and 1 mole Li=6.02 X 10^23
Empirical formula
simplest whole number ratio of atoms in a compound
All ionic compounds are written as
empirical formulas
- Ex. Of empirical formula- NaCl, CaCO3, CH2O
Sometimes a formula can be both
molecular and empirical if the molecular formula cannot be further reduced. Ex. H2O
Subscripts can represent
atomic ratio or molar ratio
Significant digits rules
numbers from the periodic table and Avogadro’s number are infinitely significant
Formula unit
Ionic compound (cation + anion, usually metal and nonmetal). Ex. NaCl, (NH4)3PO4
Molecule
covalent compound (2 nonmetals)
Atoms
elements
Things that go together (within the same fraction)
- Given value and 1
- Molar mass and 1 mol__
- Avogadro’s number particle and 1 mol__
- 1 mol element and atomic mass of the element
Moles ⇐⇒Mass/grams
- How to recognize: The problem will say “mass” (or “grams”) and “mols” and will ask you to change from mass to moles or mols to mass.
- How to Solve:
1. Write out compound
2. List out each element
3. Find the molar mass of the compound (see problem type D and E for how to do this) make sure you label is g/particle
4. Next is the calculation
5. Put the value given over one
6. For these problems, there will always be a 1 in front of “mole” (if the given value is in moles, there will be a “1 mole __” in the denominator of what you’re multiplying by
7. Make sure the label of the numerator of the first value and the label of the denominator cancel out, so put the value you got in step 3, in whichever place allows you to do so.
8. Final label should either be g or moles, depending on what the question is asking for
Moles⇐⇒Particles
- How to recognize: you will see the words “mole” and the type of particle “fu/molecules/atoms” in the problem
- How to solve:
1. Put given value over 1
2. The only two vales that can be in the second fraction are “1 mole__” and “6.02 x 1023 fu/molec/atoms___(Avogadro’s number”
3. Put as denominator/numerators so the labels cancel out
4. Final label will be either moles or fu/molec/atoms depending on what the question asks for
Particles⇐⇒g/mass
- How to recognize: The problem will have particles and g/mass in it and will be asking you to convert between the two
- How to solve:
1. Put the given value over 1
2. Find molar mass of compound by listing all the elements in the compound and multiplying their atomic mass by their subscripts
3. Goal is to multiply to cancel out the labels
4. Unless it’s the given value, there will always only be a 1 in front of “mole”
5. If the problem gives you a value in grams, the labels should go as follows: (given value g___/1) X (1 mol__/molar mass) X (avagadros number particle___/1 mol__)
6. If the problem’s given value is in particles, the labels should go as follows: (given value partice__/1) X (1 mol__/avagadros number particle__) X (molar mass/1 mole__)
7. Final label will either be g or particles depending on what the question asks for
Formula mass of compounds
- How to recognize: will ask for the formula mass
- How to Solve:
1. Write the compound
2. List every element in the compound individually
3. For each element, multiply their atomic mass to the nearest tenth by the number of particles
4. Atomic mass- number on the period table
5. Number of particles- subscript of the atom
6. Don’t forget, subscripts apply to all elements if they’re in parenthesis, so in (OH)2 there are 2 partilces of O and 2 particles of H
7. Take the numbers you got from step 3 and add them together
8. Use the Label “amu/particle (fu, molec, or atom)”
Molar mass
- How to recognize: will ask for the molar mass
- How to solve:
1. Follow the exact same steps as D
2. Label it as g/mol instead of amu/particle
Percent compositions with formula of compound
- How to recognize: will ask for percent composition and five the formula of the compound
- How to Solve:
1. Take the mass of one element in the compound (one of the numbers you add to get the total in D and E) and divide it by the total mass
2. Multiply it by 100
Percent composition of elements of mass of a compound
- How to recognize: will ask for percent composition and give the mass of the sample of the compound
- How to solve:
1. Take the g of each element, divide them by the total mass of the compound and multiply
2. You should have more than one answer- a percent for each element
3. If it gives you the percent of the element and asks for the grams (like number 4), divide the percent by 100 and multiply by the total mass of the compound
4. You might have to subtract the masses of given elements from the total mass of the compound to find the mass of an element if its not given
Determine Empirical formula
- How to recognize: will ask for the empirical formula
- How to solve:
1. For each element, write the mass of the element/1 and multiply by 1 mol element/atomic mass and label them g element
2. Take those numbers, divide each one by the smallest one out of all of them
3. If numbers are in whole number ratios, use that as the subscripts in your formula, if not, multiply them to get the smallest whole number ratio and use that
4. Problems like 13 and 15 give the percent compositions of each element but not the mass. In this case, use percent composition (percent, divide by 100, multiply by total mass of compound/denominator) to find the masses, then use those as you normally would
Determine the molecular formula
- How to recognize: will ask for molecular formula
- How to Solve:
1. Follow the exact same steps from H to determine the empirical formula
2. Use the empirical formula to find the mass of the empirical formula and see if it matches the mass of the compound
3. To do this, take the mass of the compound and divide it by the mass you got
4. Multiply the subscripts of each of the elements by that number to get the molecular formula
5. If this number is one, you already have the molecular formula (empirical and molecular are the same in this case)
