Stoichiometry

Question 1

Density of material is (lesser, greater, same) for an object on the moon as it is on Earth.

Answer 1

Same. Density of a material or solution is the mass of the sample divided by the volume of the sample. Mass and therefore density is a QUANTITY.

Question 2

Define specific gravity

Answer 2

Density of a material relative to the density of water.

Question 3

To solve for density dealing with a solution, what are the units?

Answer 3

Mass solution divided by volume solution.

Question 4

How to solve mass percent of a solute in solution?

Answer 4

Mass solute divided by mass solution.

Question 5

What is the product of the density and mass percent?

Answer 5

Mass solute divided by volume solution

Question 6

When an object floats in a liquid medium, the density is (less, greater, same) of the medium surrounding it?

Answer 6

Less. An ice cube floats in water because it is less dense.

Question 7

When an object sinks in water, it is (less, greater, same) than water.

Answer 7

Greater. Metal objects sink in water because they are less dense.

Question 8

All conversions involving reactants and/or products must pass through ____.

Answer 8

Moles!!! See example 1.8.

Question 9

Definition of moles?

Answer 9

The amount of chemical substance that contains as many representative particles (eg. atoms, molecules, ions) as there are atoms in 12 g of carbon. Avo number = 6.022*10^23.

Question 10

What is the difference between atomic mass and molecular mass?

Answer 10

Mass of elements versus mass of compounds.

Question 11

What is elemental analysis? What must be true for this analysis to work? What process does it involve?

Answer 11

Elemental analysis determines the atomic composition of unknown molecules. We assume that all molecules of the same substance combine atoms the same way, structural uniformity. The process involves oxidizing an unknown compound completely and collecting the products.

Question 12

Definition of empirical formula.

Answer 12

The simplest whole number ratio of atoms in a molecule.

Question 13

Formula for mass percent.

Answer 13

mass percent = (mass of atoms/ mass compounds) * 100%

Question 14

Describe the mass of multiple proportions.

Answer 14

If two elements form more than one compound between them, then the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers.

Question 15

Definition of molecular formula.

Answer 15

The actual mole ratio of the elements within the compound.

Question 16

What is combustion analysis? How is it accomplished?

Answer 16

Determining the mass percent of each component element in an unknown compound. It is accomplished by oxidizing the unknown with excess oxygen ( to ensure complete combustion), followed by the separation and collection of all the oxidized products.

Question 17

What is produced when a hydrocarbon is oxidized?

Answer 17

CO2 and water.

Question 18

With combustion analysis, how is CO2 and water separated?

Answer 18

Passage through hygroscopic salt of a known mass. The salt absorbs water, and thus increases in mass. This salt must NOT react with CO2. Calcium chloride and magnesium sulfate are good salts to use.

Question 19

In combustion analysis, how is the mass percent of CO2 determined?

Answer 19

Once water is absorbed, the remaining gas is passed across a sample of KOH of known mass. KOH reacts with CO2 to form KHCO3. The hydroxide salt absorbs the CO2 and thus increases in mass.

Question 20

Aside from combustion analysis, what is the other method to determine the mass percent of a compound?

Answer 20

By lowering the temperature, CO2 and water can be converted to their solid states. Because solids do not flow, they could be collected easily.

Question 21

Concentration units for: molarity?

Answer 21

Moles solute per liter of solution.

Question 22

Concentration units for molality?

Answer 22

Moles solute per kilogram of solvent.

Question 23

Concentration units for mass percent?

Answer 23

Mass solute per mass of solution. Should be calculated in grams.

Question 24

Concentration units for density?

Answer 24

Mass solution per volume solution.

Question 25

Define molality.

Answer 25

Molality (m) is the concentration of a fluid solution defined as the moles of a solute per kilogram of solvent.

Question 26

Define molarity.

Answer 26

Molarity (M) is the concentration of a fluid solution defined as the moles of a solute per volume of solution.

Question 27

Which of the following remains constant at temperature changes: molarity, molality, mass percent, density.

Answer 27

Molality and mass percent.

Question 28

How do you calculate the total mass of a solution?

Answer 28

The total mass of a solution is the sum of the mass of solute and mass of solvent. This is used for Mass percent in solution questions .

Question 29

Define density.

Answer 29

Density (p) is the concentration of fluid solution defined as the mass of solution per volume of solution. The density of a solution VARIES with temperature. The density of a homogeneous solution is uniform throughout, so a small sample of the solution has the same density as the entire solution.

Question 30

The moles of solute is (less, greater, the same) when a solvent is added.

Answer 30

The moles of solute should remain constant when a solvent is added. MiVi = M2V2. This is a dilution question.

Question 31

Dilution: what is another way to say factor of three?

Answer 31

Threefold dilution.

Question 32

A solution can be diluted either by addition of solvent or another solution to it. The adding of pure solvent is known as simple dilution. How do we find the the final concentration if a different solution is added?

Answer 32

The final concentration lies somewhere between the two initial concentration values. The final concentration is the weighted average of the initial concentrations.

Question 33

Equal volumes of two solutions are mixed together. How do you find the final concentration.

Answer 33

Because equal volumes are mixed, the final concentration will be an average of the two initial concentrations. Divide by 2.

Question 34

Define Beers Law.

Answer 34

The Beer-Lambert law (or Beer’s law) is the linear relationship between absorbance and concentration of an absorbing species.

Question 35

Equation for Beer’s Law?

Answer 35

Absorbance = εCl; ε = constant for the solute at λmax ( the λ at greatest absorbance); C = concentration; l = length of cuvett. Note that absorbance is proportional to concentration.

Question 36

What is the law of conservation of matter?

Answer 36

The number of atoms must be identical on each side of the reaction. When balancing equations, start with the compound whose atoms are least present in the reaction.

Question 37

Define limiting reagents.

Answer 37

The reactant with the lowest ratio of actual moles to needed moles (based on the balanced equation). To determine the limiting reagent, the amount of all reactants and the mole ratio of the reactants must be known.

Question 38

What is a precipitation/double displacement reaction?

Answer 38

A reaction that involves two aqueous salts being added together to form spectator ions and a solid salt precipitate that drops out of the solution.

Question 39

What is an acid base reaction?

Answer 39

A reaction between an acid (a proton donor) and a base (a proton acceptor) forms a neutral salt and water.

Question 40

What is a composition reaction?

Answer 40

A composition reaction involves the combining of reactants to form a product. Entropy decreases and more bonds are formed than are broken in composition reactions.
Ex. PCl3 + Cl2 –> PCl5

Question 41

What is a decomposition reaction?

Answer 41

A decomposition reaction is the opposite of a composition reaction. Reactants decompose to form multiple products.

Question 42

What is an oxidation/reduction reaction?

Answer 42

An oxidation reduction reaction involves the transfer of electrons from one atom to another.

Loss of electrons is defined as oxidation, while gain of electrons is defined as reduction. LEO GER

Question 43

What is a combustion reaction?

Answer 43

Combustion reactions are a special case of oxidation-reduction reactions, where the oxidizing agent is oxygen gas, and the products are oxides and water.

Question 44

What is percent yield?

Answer 44

Percent yield is the percent ratio of actual yield to the theoretical yield. It is calculated to be the experimental yield divided by theoretical yield multiplied by 100%

Question 45

What is convection?

Answer 45

When a fluid, such as air or a liquid, is heated and then travels away from the source, it carries the thermal energy along. This type of heat transfer is called convection. The fluid above a hot surface expands, becomes less dense, and rises.

Question 46

What type of liquid should you choose for a Dumas experiment, where you are trying to find the molecular mass/weight of an unknown substance?

Answer 46

Choose one with a high boiling point (so not all the liquid evaporates and high molecular mass ( because errors will be less significant in a larger number than smaller number.

Question 47

What is the oxidation state of Fe in Fe2O3?

Answer 47

There are two Fe atoms and three O atoms in Fe2O3.

Therefore, 2X + 3(-2) = 0; 2X = +6 ; X = +3.