COUNTING UNITS FOR…….. atoms

These slides will help you navigate the next section of the Counting Units Lab – Experiment #5/Unit 2 (B): Mole System. Have a sheet of paper on which to answer questions and work problems. You will turn in your work at the end of the class period.

Recall Part (A) Counting Unit – Model System: 1.We developed a relative mass scale for seeds 2.We determined, and named, a “counting unit” by weighing out one relative mass (in grams) then counting the number of seeds in that g-relative mass. 3.The counting unit was the same for each seed-type, regardless of its size. 4.Moreover, the counting unit describes a relationship between the relative mass and actual mass of a seed (relative mass/actual mass).

5.The counting unit is a NUMBER ….. our counting unit was ________ (#), and was named “_______________________”. 6.By knowing the relative mass of a seed-type, and the # of seeds in a counting unit, we found that we could easily measure out large numbers of seeds, e.g., if the relative mass of corn seeds was 6.0 g, and the counting unit was 50 seeds, then we could “measure” 500 seeds simply by weighing out ________ g. ( yes, the answer is 60.0 g ) !

Chemists also use a counting unit: to “measure” out numbers of atoms. This counting unit is called the mole. Just like all counting units, it is a NUMBER: 6.02 x This is quite a large number: write it out on your paper. I’ll wait………………………………………………………

Check it out…. Just a couple more: If 10 billion chickens each laid 10 eggs per day, it would take… > 10 billion years to produce 1 mole of eggs… One mole of those chicken eggs would fill all the oceans on earth…. 30 million times….

The Periodic Table contains the relative masses of all the atoms…… Masses represent masses of atoms and are given in atomic mass units (amu). Our mass of one grain of rice corresponds to the atomic mass unit of the periodic table.

The relative masses on the periodic table are based on the “standard”: the carbon-12 isotope. The masses of atoms of all the other elements are “relative” to the mass of an atom of carbon-12. An atom of sulfur has a mass of ______ amu. It’s mass is ______ X the mass of an atom of hydrogen.

Why is the mole counting unit so very large ? (6.02 x ) Because atoms are so very small, and it takes so very, very, very many of them to give the relative mass, in grams. We chemists measure atoms in grams.. When we weigh out a relative mass of atoms in grams, we are measuring moles of atoms !! A relative mass on the periodic table in amu = mass of one atom. A relative mass on the periodic table in grams = mass of one mole of atoms.

Measured mass of one….. Unit of mass We call it … Atom Atomic mass unit (amu) Atomic mass Mole Grams (g) Molar mass Atomic mass unit – used to describe masses of single atoms (protons + neutrons). Grams – used to describe masses of moles of atoms.

Recall – from our Model System: (a) the relative mass of any seed-type contained the same number of seeds (the counting unit) And so, in the Mole system: (b) the relative mass of any atom-type contains the same number of atoms (6.02 x ) Where does the counting unit for atoms (Avogadro’s number = 6.02 x ) come from…………………………………………

Our model seed system showed us that the counting unit describes a relationship between the relative mass and the actual mass : relative mass = counting unit actual mass Let’s start with Hydrogen: How many atoms are in one relative mass of hydrogen (1.0 g)? 1. Relative mass = 1.0 gActual mass = 1.7 x g (H has 1 p + ) 2. relative mass = 1.0 g___________ = 5.9 x atoms actual mass 1.7 x g/atom Mass of 1 proton (p + ): 1.67 x gMass of 1 neutron (n 0 ): 1.67 x g

Turn to page 2 of Exp’t 5/Unit 2(B) handout and complete table: Use masses of protons and neutrons given to sum up actual mass for each atom. Atom# protons# neutronsmass of atom C 6 6 N 7 7 O 8 8 Cl 9 10 Ar Complete the table for the elements given, then add 2 more of your choice from the periodic table! Check answers on the next slide…………………………………………

ElementActual mass (g) Relative mass scale #1 (amu) Relative mass scale #2 (g) # atoms in one relative mass (rel mass/actual mass) H 1.7 x amu 1.0 g 5.9 x C2.00 x amu g 6.01 x O2.67 x amu g 5.99 x Cu1.05 x amu g 6.05 x Pb3.46 x amu g 5.99 x Avogadro’s number, the mole, 6.02 x converts the masses of atoms on the periodic table (amu) into masses of moles of atoms (g) !

Some useful conversion factors: 1 mole = 6.02 x atoms 1 mol = relative mass (g) e.g.,C12.01 g/mol atoms Na 1 mol atoms/23.0 g Solve:How many grams in 0.50 mol S? 0.50 mol 32.1 g_______ = 16.1 g S 1 mol S atoms

Time for practice (use dimensional analysis to solve): 1.What is the relative mass (in grams) of phosphorous? 2.The mass of one atom of Mg is _____________; the mass of one mole of Mg atoms is ____________. 3.What is the mass, in grams, of 3.0 mol of aluminum? 4.Determine the number of atoms in 25.5 g of silver. 5.How many atoms are in 3.45 mol of cobalt? What is the mass of this number of atoms? 6.If you use 4.3 g of sodium in an experiment, how many moles will you use? 7.Determine the number of moles in 125 g of zinc. 8.How many atoms are in 13.4 g of copper? Compare this to the number of atoms in 13.4 g of silver. (check key on the board to confirm your answers)

When you have finished the practice problems, turn in your paper and begin the Calculations and Questions for part (B) of Exp’t 2.5.