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## PowerPoint Slideshow about 'Chemical Quantities Chemistry Tracy Bonza Sequoyah High School' - fiona

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### Empirical Formula

How do you measure how much?

- You can measure mass,
- or volume,
- or you can count pieces.
- We measure mass in grams.
- We measure volume in liters.
- We count pieces in MOLES.

Moles

- Defined as the number of carbon atoms in exactly 12 grams of carbon-12.
- 1 mole is 6.02 x 1023 particles.
- Treat it like a very large dozen
- 6.02 x 1023 is called Avogadro's number.

Measuring Moles

- Remember relative atomic mass?
- The amu was one twelfth the mass of a carbon 12 atom.
- Since the mole is the number of atoms in 12 grams of carbon-12,
- the decimal number on the periodic table is also the mass of 1 mole of those atoms in grams.

Representative particles

- How many oxygen atoms in the following?
- CaCO3
- Al2(SO4)3

- How many ions in the following?
- CaCl2
- NaOH
- Al2(SO4)3

Molar Mass

- The mass of 1 mole of an element in grams.
- 12.01 grams of carbon has the same number of pieces as 1.008 grams of hydrogen and 55.85 grams of iron.
- We can write this as 12.01 g C = 1 mole
- We can count things by weighing them.

Zinc

Neon

39g

65.4g

20.2g

What is the molar mass of:What does that mean?

It means that 39g of K is 1 mole of potassium

How many particles of potassium is that?

6.02 x 10 23

See…….we can count particles without counting them

……….we weighed them!!!

What about compounds?

- in 1 mole of H2O molecules there are two moles of H atoms and 1 mole of O atoms
- To find the mass of one mole of a compound
- determine the moles of the elements they have
- Find out how much they would weigh
- add them up

What about compounds?

- What is the mass of one mole of CH4?
- 1 mole of C = 12.01 g
- 4 mole of H x 1.01 g = 4.04g
- 1 mole CH4 = 12.01 + 4.04 = 16.05g
- The Molar mass of CH4 is 16.05g
- The mass of one mole of a molecular compound.

Molar Mass

- The generic term for the mass of one mole.
- The same as gram molecular mass (for molecules/covalent compounds), gram formula mass (for formula units/ionic compounds), and gram atomic mass (for atoms).
- I will use the generic molar mass (sometimes I slip up and call it gram formula mass so work with me here!!!!!)

Examples

- Calculate the molar mass of the following and tell me what type it is.
- Na2S
- N2O4
- C
- Ca(NO3)2
- C6H12O6
- (NH4)3PO4

Molar Mass

- The number of grams of 1 mole of atoms, ions, or molecules.
- We can make conversion factors from these.
- To change grams of a compound to moles of a compound.

For example

- How many moles is 5.69 g of NaOH?

For example

- How many moles is 5.69 g of NaOH?

For example

- How many moles is 5.69 g of NaOH?

- need to change grams to moles
- for NaOH
- 1mole Na = 22.99g 1 mol O = 16.00 g 1 mole of H = 1.01 g

For example

- How many moles is 5.69 g of NaOH?

- need to change grams to moles
- for NaOH
- 1mole Na = 22.99g 1 mol O = 16.00 g 1 mole of H = 1.01 g
- 1 mole NaOH = 40.00 g

For example

- How many moles is 5.69 g of NaOH?

- need to change grams to moles
- for NaOH
- 1mole Na = 22.99g 1 mol O = 16.00 g 1 mole of H = 1.01 g
- 1 mole NaOH = 40.00 g

For example

- How many moles is 5.69 g of NaOH?

- need to change grams to moles
- for NaOH
- 1mole Na = 22.99g 1 mol O = 16.00 g 1 mole of H = 1.01 g
- 1 mole NaOH = 40.00 g

Examples

- How much would 2.34 moles of carbon weigh, in grams?
- How many moles of magnesium in 24.31 g of Mg?
- How many moles is 4.56 g of CO2 ?
- How many grams is 9.87 moles of H2O?

Moles and Molecules

- 6.02 x 1023 =1 mole too!
- So we can convert from moles to molecules too!
- Try it:
- How many molecules are there in 3.6 moles of sodium hydroxide?
- How many moles is 3.6x 1034 molecules of gold?

3 Step Problems

- Converting from grams to molecules/atoms and molecules/atoms to grams
- How many atoms of silver are in 2.3g of silver?

Examples

- How many molecules in 6.8 g of CH4?
- 49 molecules of C6H12O6 weighs how much?
- How many atoms of lithium in 1.00 g of Li?
- How much would 3.45 x 1022 atoms of U weigh?

Gases

- Many of the chemicals we deal with are gases.
- They are difficult to weigh.
- Need to know how many moles of gas we have.
- Two things effect the volume of a gas
- Temperature and pressure
- Compare at the same temp. and pressure.

Standard Temperature and Pressure

- 0ºC and 1 atm pressure
- abbreviated STP
- At STP 1 mole of gas occupies 22.4 L
- Called the molar volume
- Avogadro’s Hypothesis - at the same temperature and pressure equal volumes of gas have the same number of particles.

Examples

- What is the volume of 4.59 mole of CO2 gas at STP?
- How many moles is 5.67 L of O2 at STP?
- What is the volume of 8.8g of CH4 gas at STP?

Density of a gas

- D = m /V
- for a gas the units will be g / L
- We can determine the density of any gas at STP if we know its formula.
- To find the density we need the mass and the volume.
- If you assume you have 1 mole than the mass is the molar mass (PT)
- At STP the volume is 22.4 L.

Examples

- Find the density of CO2at STP.
- Find the density of CH4 at STP.

The other way

- Given the density, we can find the molar mass of the gas.
- Again, pretend you have a mole at STP, so V = 22.4 L.
- m = D x V
- m is the mass of 1 mole, since you have 22.4 L of the stuff.
- What is the molar mass of a gas with a density of 1.964 g/L?
- 2.86 g/L?

We have learned how to

- change moles to grams
- moles to atoms
- moles to formula units
- moles to molecules
- moles to liters
- molecules to atoms
- formula units to atoms
- formula units to ions

Moles

Percent Composition

- Like all percents
- Part x 100 % whole
- Find the mass of each component,
- divide by the total mass.

Example

- Calculate the percent composition of a compound that is 29.0 g of Ag with 4.30 g of S.

Getting it from the formula

- If we know the formula, assume you have 1 mole.
- Then you know the pieces and the whole.

Examples

- Calculate the percent composition of C2H4?
- Aluminum carbonate.

From percentage to formula

The Empirical Formula

- The lowest whole number ratio of elements in a compound.
- The molecular formula the actual ration of elements in a compound.
- The two can be the same.
- CH2 empirical formula
- C2H4 molecular formula
- C3H6 molecular formula
- H2O both

Calculating Empirical

- Just find the lowest whole number ratio
- C6H12O6
- CH4N
- It is not just the ratio of atoms, it is also the ratio of moles of atoms.
- In 1 mole of CO2there is 1 mole of carbon and 2 moles of oxygen.
- In one molecule of CO2 there is 1 atom of C and 2 atoms of O.

Calculating Empirical

- Means we can get ratio from percent composition.
- Assume you have a 100 g.
- The percentages become grams.
- Can turn grams to moles.
- Find lowest whole number ratio by dividing by the smallest.

Example

- Calculate the empirical formula of a compound composed of 38.67 % C, 16.22 % H, and 45.11 %N.
- Assume 100 g so
- 38.67 g C x 1mol C = 3.220 mole C 12.01 gC
- 16.22 g H x 1mol H = 16.09 mole H 1.01 gH
- 45.11 g N x 1mol N = 3.219 mole N 14.01 gN

Example

- The ratio is 3.220 mol C = 1 mol C 3.219 molN 1 mol N
- The ratio is 16.09 mol H = 5 mol H 3.219 molN 1 mol N
- C1H5N1
- A compound is 43.64 % P and 56.36 % O. What is the empirical formula?
- Caffeine is 49.48% C, 5.15% H, 28.87% N and 16.49% O. What is its empirical formula?

Empirical to molecular

- Since the empirical formula is the lowest ratio the actual molecule would weigh more.
- By a whole number multiple.
- Divide the actual molar mass by the the mass of one mole of the empirical formula.
- Caffeine has a molar mass of 194 g. what is its molecular mass?

Example

- A compound is known to be composed of 71.65 % Cl, 24.27% C and 4.07% H. Its molar mas is known (from gas density) is known to be 98.96 g. What is its molecular formula?

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