stoichiometry_soln

Question 1

Define stoichiometric point.

Answer 1

Stoichiometric point in a chemical reaction pertains to the conditions when at least two reagents are present in relative quantities that match the balanced chemical equation. As a result, once the reaction is complete, neither of these reagents will remain in excess.

Question 2

Define limiting reagent.

Answer 2

Limiting reagent in a chemical reaction is the reagent which will be completely consumed when the reaction proceeds to completion. As such, it is this reagent which defines how much product will form.

Question 3

Define percent yield.

Answer 3

Percent yield is a ratio which compares how much product is formed or recovered versus how much would be formed theoretically if all of the limiting reagent is completely reacted.

This can only be calculated if you have information about how much product is formed or recovered. It is calculated using the equation: mass of product recovered/ mass of product theoretically formed x 100%

Question 4

Define percent composition in a mixture.

Answer 4

Percent composition in a mixture describes the ratio of how much chemical of interest is present in an overall mixture.

Typical examples are how much iron is present in iron ore or how much aspirin is present in an aspirin tablet.

Percent composition can be often be determined by finding the overall mass of the mixture and then subject the mixture to a chemical reaction specific to the chemical of interest. By finding how much product is formed or how much reagent is required, one can estimate how much original chemical was present. Percent composition is calculated using the equation: mass of chemical of interest/ mass of overall mixture x 100%.

Question 5

How does stoichoimetric point compare to equivalence point?

Answer 5

Essentially the stoichiometric point and equivalence point represent the same conditions. These are conditions when at least two reagents are present in relative quantities that match the balanced chemical equation.

The words “stoichiometric point” represent a more general example, whereas “equivalence point” refers to the stoichiometric point in the context of a titration. That is, one reagent (the titrant) is added to another (the analyte) in a step wise fashion with goal of finding the stoichiometric point. This information can be used to determine the original quantity of the analyte.

Question 6

Define mole ratio in a balanced equation.

Answer 6

The mole ratio indicates the proportions by which reagents react or products are formed.

For example the following balanced equation:

2H₂ + O₂ → 2H₂O

shows a mole ration of 1 mol O₂/ 2 mol H₂O and 2 mol H₂/ 2 mol H₂O.

Question 7

Define molar concentration.

Answer 7

Molar concentration is the amount of solute ( in moles) per liter of solution.

Question 8

5.00 mL of 0.100 M NaCl solution is diluted to 100.00 mL. How many fold dilution is this? What is the new concentration?

Answer 8

A 5 to 100 mL dilution is a 20 fold dilution or a 20 x dilution (100/5 = 20). The new concentration will be 20 x smaller than the original concentration. Therefore: 0.100 M /20 = 0.00500 M.

Question 9

10.00 mL of 0.125 M Na₂S₂O₃ solution is diluted to 250.00 mL. How many fold dilution is this? What is the new concentration?

Answer 9

A 10 to 250 mL dilution is a 25 fold dilution or a 25 x dilution (250/10 = 25). The new concentration will be 25 x smaller than the original concentration. Therefore: 0.125 M /25 = 0.00500 M.

Question 10

Given 0.100 M NaCl. What are the concentrations of Na⁺ ions and Cl^- ions?

Answer 10

Because there are 1 mol Na⁺ /1 mol NaCl and 1 mol Cl^-/ 1 mol NaCl, then [Na⁺] = 0.100 M and [Cl^-] = 0.100 M.

Question 11

Given 0.125 M Na₂S₂O₃. What are the concentrations of Na⁺ ions and S₂O₃^2- ions?

Answer 11

Because there are 2 mol Na⁺ /1 mol Na₂S₂O₃ and 1 mol S₂O₃^2-/ 1 mol Na₂S₂O₃, then [Na⁺] = 0.250 M and [S₂O₃^2-] = 0.125 M.