unit 3: quantities in chemical reactions

accuracy

How close a given quantity is to the accepted value.

Precision

How exact a measurement is. How close measurements are to each other.

sig figs

The number of digits used to express a number to the required degree of precision 

sig fig rules

 

All Non zero numbers are significant

All zeros located between non zeros are significant.

Zeros to the left of non zeros are not significant.

Zeros to the right of non zeros are significant only when decimal present

Exact numbers, for example the number of meters in a kilometer or counted numbers have infinite sig figs.

scientific notation and sig figs

Is used to express very large or very small numbers.

E.g 10 492 000 m

1.0492 x 10⁷ m

Can use scientific notation to round to the correct number of sig figs:

3 498 to 2 sig figs = 3.5 x 10³

multiplying/dividing sig figs

1.  Multiplying and Dividing

Your answer should have the same total digits as the number in the question with the fewest total significant figures.

Ex. 2.34 cm x 1.5 cm

= 3.5cm²

2.  Adding and Subtracting 

Express your answer with the smallest number of Decimal Places used in the question.

Ex. 4.35 kg + 0.346kg

= 4.70 kg

3.  Rounding 

1. f the number after your last significant figure is more than five then round up one number.

Ex. 2. 346 to three sig figs = 2.35

b)  If the number after your last significant figure is less than five then leave the number as is.

Ex. 5.73 to two sig figs = 5.7

Rounding pt 2

If the number ends in five after the last significant figure (with nothing after the 5), this is done two ways.

3. If the last significant figure is odd, round up.

Ex. 18.35 (3 sig figs) = 18.4

4. If the last significant figure is even,leave it as is.

Ex. 18.25 (3 sig figs) = 18.2

atomic masses

• What do the atomic masses on the periodic table represent?

  • E.g.  C has an atomic mass of 12.01 amu (atomic mass units).

• What is the actual mass of one C atom? 1.99 × 10^-23

molecules vs formula units

• Covalent compounds (e.g. H₂O) are composed of molecules

• Ionic compounds (e.g. NaCl) are composed of formula units

the mole

• Contains 6.02 x 10²³ particles 

n = moles (mol)

N = number of particles

N_A= Avogadro's number 

(6.02 x 10²³ particles/mol)

n = N/N_A

molar mass

Mass of 1 mole of substance (6.02x10²³ particles)

• Units: g/mol

mass to moles

n = moles (mol)

m = mass (g)

M = molar mass (g/mol)

n = m/M

percent composition

for actual masses:

%X = m_x/m_total x 100

for molar masses:

%X = Mx / Mtotal x 100

hydrates

to find percent of water:

1. find mass of just water

2. use mass of just water, hydrate, to find % of water

to find the formula of the hydrate:

1. find moles of water

2. find moles of anhydrous

3. x = mol H2O / mol anhydrous

hydrates definition

An ionic compound that has a specific number of water molecules bound to each formula unit.

Solids only - water molecules released when aqueous

anhydrous

An ionic compound that has no attached water molecules.

Eg. ZnSO₄

empirical formula

The lowest whole number ratio of atoms, for example: CH₂O 

molecular formula

The actual number of atoms in a compound, for example:

CH₂O or C₆H₁₂C₆

to get empirical formula

1. % to mass: Assume 100g total

2. Mass to moles: Convert to moles

3. Divide by the smallest: Divide each value by the smallest number of moles to find the ratio 

Multiply until whole: multiply the numbers in the ratio to get whole number subscripts

to get molecular formula

1. Find the molar mass of the empirical formula

2. Divide actual molar mass (provided in the question) by empirical molar mass  to get multiplication factor X

3. Multiply the subscripts in the empirical formula by this multiplication factor (X, the answer from step two )

balanced chemical equations for stoichiometry

4 NH_3(aq) + 5O_2(g) →  6H₂O _(l) + 4NO_(g)

• The balanced chemical equation has the same number of each type of atom on each side (The Law of Conservation of Mass)

• For every 4 mol NH₃ used, we need 5 mol O₂, which produces 6 mol H₂O and 4 mol NO

• This ratio does NOT work for direct conversion from mass to mass because each compound has a different molar mass

mole ratio

• These problems require a balanced chemical equation.

The fraction that we use to determine the new number of moles is called a mole ratio.

stoich mass to mass

1. Balance equation

2. Convert to moles

3. Mole ratio

4. Convert from moles to desired quantity (mass, particles etc)

percent yield

• Many reactions do not go to completion

• Not all reactants will be converted to products

sources of error 

1. Not all product recovered

• Not all the product makes it onto the scale to be weighed (NOT due to a spill but something that happens even if the procedure is followed perfectly)

2. Equipment limitations

• Even if functioning perfectly, all devices have limits in accuracy (a pipette measurement is accurate to 2 decimal places, a graduated cylinder to 1 decimal place)

3. Reactant impurities

• If a reactant purchased from the manufacturer is 95% pure (higher purity is more expensive), less starting material is available to react, and less product will be made (your calculations are based on 100% purity).

Side reactions occur

• One or more of the starting materials react to create a different product.  Therefore, less reactant is available to make the desired product.  The unwanted product could be lighter or heavier than the desired product.

5. Product is not dry

• Water is also being weighed or a hydrate has formed making the product appear heavier 

limiting reagent

• The reactant that limits the reaction and produces the least amount of product

excess reagent

• There will be some of the other reactant left over and it is called the “excess reagent”