Intro to Chapter 7 Formula Mass and Moles

Atomic Mass & Formula Mass Atomic mass mass of an element; measured in amu; found on p. table Na = Cl = Formula mass mass of a compound; measured in amu; calculated sum of the atomic masses of elements NaCl =

Formula Mass, contd. H 2 O = C 6 H 12 O 6 =

Formula Mass, contd. Fe(NO 3 ) 3 =

The Mole  a chemist’s counting unit  a quantity of matter equal to _________ particles “Avogadro’s #” 1 mole Na = __________particles 1 mole Cl = __________particles 1 mole NaCl = _________particles The quantity of 1 mole is NOT affected by the identity of the substance.

The Mole, cont’d.  the mass of matter equal to the atomic mass or formula mass of a substance, measured in “grams”. 1 mole Na = __________ g 1 mole Cl = __________ g 1 mole NaCl = _________g The mass of 1 mole IS affected by the identity of the substance.

In Summary, 1 mole Na = 6.02 x particles = g 1 mole Cl = 6.02 x particles = g 1 mole NaCl = 6.02 x particles= g Even though they weigh different amounts, the quantity of matter remains the same!!!

Mole Coversions…. Converting between units Use the factor-label method of problem-solving! * Multiply the given value by a conversion factor that will allow the units of the original measurement to cancel out. Multiply the numerators. Divide by the denominators. * Your conversion factor shows how two measured numbers are related to each other. 1 mole 6.02 x particles # grams (p. table)

Examples: 1. How many particles are present in 0.75 moles of carbon dioxide, CO 2 ? 2. How many moles contain 4.88 x particles of Phosphorus?

3. How many grams of Magnesium, Mg, are equal to 3.25 moles? 4. If you had g of Al 2 O 3, how many moles do you have?

5. A sample of Iron contains 5.22 x particles. How much will this sample of iron weigh? 6. How many particles are in a 250. g sample of table sugar, C 12 H 22 O 11 ?

Solutions  A solution is a HOMOGENEOUS mixture. - uniform composition, all parts of mixture are same.  A solution is made of 2 parts: 1. SOLVENT – present in the largeramount. 2. SOLUTE – present in the smaller amount.

Mass is NOT gained nor lost when a solution is formed. The formation of a solution is a PHYSICAL change. The composition of the substances are not chemically altered. The process of making a solution is called DISSOLVING.

Solutions can Exist in Any Phase Liquid Solutions salt water (salt + water) vinegar (acetic acid + water) Solid Solutions “alloys” – mixtures of metals (brass = copper + zinc) Gas solutions air (N 2, O 2, H 2, CO 2 …)

Solutions in which water is the solvent are called AQUEOUS solutions. Solutes can be classified as molecular or ionic. Molecular = composed of all NONMETALLIC elements. (C 6 H 12 O 6, CO 2, NH 3 ) Ionic = composed of at least one METALLIC element or HYDROGEN is the first element. (NaCl, HCl, MgSO 4, H 2 SO 4 )

Electrolytes vs. NonElectrolytes Electrolyte - solutions that contain an IONIC solute; can conduct an electrical current. NonElectrolyte – solutions that contain a MOLECULAR solute; cannot conduct electrical current. Demo drawing:

Solubility the amount of solute that can dissolve in a given amount of solvent at a given temperature. A “SOLUBLE” substance is able to dissolve very easily in a given solvent. An “INSOLUBLE” substance is not able to dissolve very easily in a given solvent. In general, solid solutes will become more soluble as temperature increases. Gaseous solutes will become more soluble as temperature decreases.

Solubility Curves A solubility curve is a line graph that shows how the solubility of a solute changes with temperature.

Any point that lies on the line represents a SATURATED solution. (max capacity) Any point that lies below the curve represents an UNSATURATED solution. (less than max capacity) Any point that lies above the curve represents a SUPERSATURATED solution. (more than max capacity)

If a saturated solution cools down, some of the solute will recrystallize and settle to the bottom of the container. The reformed solid is called a precipitate.

For example: If this solution cools from 70 °C to 30 °C, _______ of the solute will recrystallize (precipitate) out of the solution.

The amount of solute that will dissolve is proportional to the amount of solvent. At 65 C, the graph shows that 120 kg of the solute will dissolve in 100 kg of solvent. How many kg of solute can dissolve in 200 kg of solvent? How many kg of solute can dissolve in 300 kg of solvent? How many kg of solute can dissolve in 50 kg of solvent?

Solution Concentration The concentration of a solution describes the amount of solute that is present in a given volume of solution. A common way of expressing concentration is through “MOLARITY”. Molarity = moles of solute Liters of solution

Examples 1. What is the Molarity of a solution that has a volume of 1.25 Liters and contains 1.44 moles of solute? 2. How many moles are dissolved in L of a solution labeled “0.50 M”?

3. A chemist dissolves 15.0 g of NaCl into some water. If the total volume of the solution is 250 mL (0.250 L), what is the Molarity of the solution?

4. A chemist has 2.00 L of a solution labeled “0.75 M KNO 3 ”. How many grams of KNO 3 is in this solution?

What causes solutions to form in the first place? A solution forms because of intermolecular attraction. “Like dissolves Like” o Polar solutes will dissolve in polar solvents. o Nonpolar solutes will dissolve in nonpolar solvents. o Polar and Nonpolar DON’T MIX!! Demo drawing:

How can we get oil and water to mix? SOAP! Soap is a unique molecule that has both a polar and nonpolar structure. The polar “head” of the soap molecule is attracted to the water molecules (polar). The nonpolar “tail” of the soap molecule is attracted to the oil molecules (nonpolar).