A sensitive balance can weight to the nearest 0.0001 g, but a typical atom has a mass of only 0.000 000 000 000 000 000 000 01 g

Is it possible to keep track of atoms by counting them? Yes, but not directly. We count them in groups. That is we count atoms in the same way we count eggs by the dozen (12), bottle rockets by the gross (144), beer by the case (24), and sheets of paper by the ream (500).

The relative mass of an atom is compared to a reference atom using an instrument called a mass spectrometer. Carbon-12 has been chosen as a reference and it is assigned a mass of exactly 12 amu. The masses of other atoms are compared relative to carbon-12.

In fact, the gram atomic mass of each element contains 6.02x10²³ atoms of that element.

Number of atoms in 12.01 grams of carbon

6.02x10²³ atoms

Amount of a substance containing 6.02x10²³ particles

Why do we need this tool (the mole)?

Chemists and many others, need to know relative amounts of substances.

FORMULA MASS:

The sum of the atomic masses, for each element, found in the compound.

Exp: Calculate the formula mass of H₂O

2 hydrogen atoms @ 1.008 amu

1 oxygen atom @ 15.999 amu

2.016+15.9999= 18.015 amu

One molecule of water has a mass of 18.015 amu

Mass in grams of one mole of any substance, element, or compound.

Has the same numerical value as formula mass.

Q: What is the mass of 6.02x10²³ H₂O molecules?

A: 18.015 g

How many H₂ molecules are there in 0.25 moles of H₂?

7.9 billion carbon atoms represents how many moles of carbon?

Know what molar mass is and how to calculate molar mass.

The molar mass of an element is equal to its atomic mass expressed in grams.

Carbon has a molar mass of 12.01 g/mol

The molar mass of a compound is equal to the sum of the atomic masses, for each element, found in the compound.

CuSO₄ has a molar mass of 159.5g/mol

63.5 + 32 + 4(16) = 159.5

What is the mass of one H₂O molecule in grams?

Know the molar volume for a gas at STP

1 mol of gas = 6.02x10²³ molecules = 22.4L

The percent composition is a list of the mass percent of each element in a compound.

Calculate the percent composition of proply chloride (C₃H₇Cl)

Frist, calculate the formula mass for the compound by adding up the masses of all the elements:

3 carbons at 12.01amu each = 36.03 amu

7 hydrogen at 1.01amu each= 7.07 amu

1 chlorine at 35.45 amu

Summing these values yield:

36.03+7.07+35.45= 78.55 amu = formula mass of C₃H₇Cl

2. For each element found in the compound set up ratio of its mass over the total mass times 100 to convert to percent.

Carbon (36.03 / 78.55)x100 = 45.91% Carbon

Hydrogen (7.07 / 78.55)x100 = 9.00% Hydrogen

Chlorine (35.45 / 78.55)x100 = 45.13% Chlorine

These values add up to a hundred percent (100.04%).

Empirical formula Simplest formula for a compound; it is the smallest whole-number ratio of the atoms present.

Molecular formula Expression of the formula for a compound; it shows the actual number of atoms of each element present in one molecule of the compound.

Calculate the empirical formula for a compound composed of 26.6% potassium, 35.4% chromium, and 38.1% oxygen.

Percent means a fraction of 100. Thus, if we assume a100 grams of sample we can then use the percent values as grams of each element. From grams we find moles of each element in the formula.

Find moles potassium.

Next find moles of chromium.

Then find moles oxygen.

Next, set up a mole ratio that relates the moles of each element to the moles of the element that is least present.

Because a chemical formula must have only whole numbers we multiply by 2 to yield: K₂Cr₂O₇ as the empirical formula.

An oxide of nitrogen gave the following analysis: 3.04g of nitrogen combined with 6.95g of oxygen. Experiments show that the molecular mass of this compound is 91.0 amu. Determine its molecular formula