2.1.9 Moles and Solutions

Question 1

What is molar mass?

Answer 1

Mass per mole, units gmol^-1;

Question 2

What does concentration indicate? Use this to produce a definition for concentration.

Answer 2

1. How much solute is dissolved in a given amount of solvent;
2. The amount of solute, in mol, dissolved per 1dm^3 (1000cm^3) of solution;

Question 3

What is the unit for concentration? For example, how many moles would there be in a solution of 2 of these units? Scale this value down to 0.25.

Answer 3

1. mol dm^-3; (mols per decimetre cubed);
2. In a solution of concentration 2 mol dm^-3, there are 2 moles of solute dissolved in every 1dm^3 (not -3) of solution;
3. Likewise, 0.25dm^3 contains 0.5 mol of dissolved solute;

Question 4

Name the equation that links concentration, volume and the amount of substance; including units and suffixes. What would this equation look like in cm^3? Why would one use this method instead?

Answer 4

n = cV(in dm^3)
n - amount of substance, mol;
c - concentration of solute (the thing being dissolved), in mol dm^-3;
V - volume of solution, in dm^3;
2. n=c(in cm^3)/1000 (as there are 1000cm^3 in a dm^3);
3. One would use this when measuring smaller quantities.

Question 5

Name two things that are special about a standard solution?

Answer 5

1. It has a known concentration;
2. They are normally utilised in titrations to determine unknown information about another substance (this is usually in the form of c=n/v).

Question 6

What is the process that one would go through to create a standard solution?

Answer 6

1. Know the concentration and volume of the solution one needs to make;
2. Work out the amount (mol) of solute needed to make;
3. Convert this mount of solute into mass, so that one knows how much to weigh out;

Question 7

What are the different types of volumetric flask one can use, and how can the tell the difference between these types? When would they use a volumetric flask?

Answer 7

1. The types of flask are called CLASSES of flask;
2. Class A measures with a higher decree of precision than class B;
3. One can tell the class of volumetric flask that they use, via markings on the side of the container that indicate a letter of class A or class B;
4. These would be utilised in titrations;

Question 8

How should one measure accurately using a volumetric flask?

Answer 8

1. Use the highest precision of class (A) available;
2. Measuring the fluid up to the line marked on the side of the container with their eyes parallel to it;
3. Measuring unto the bottom of the meniscus;

Question 9

What is the 5 stage process that one would go through, to prepare a standard solution?

Answer 9

1. Use the ‘weigh by difference’ method, to weight out the solute;
2. Completely dissolve the solute in a BEAKER; transfer the solution to the flask and rinse the beaker repeatedly, using more solvent, adding rinsings to the flask;
3. Add solvent to the flask, but do not fill it all the way up to the graduation line;
4. Carefully add solvent drop by drop up to the line on the flask, until the bottom of the meniscus sits exactly on the graduation mark - if the solution goes over the line, it must be disposed of and started again;
5. Mix the solution thoroughly, by inverting the flask several times;

Question 10

What is the graduation line?

Answer 10

The line on the volumetric flask, that the standard solution must be measured up to.

Question 11

How would one find the mass of KOH needed to prepare 250cm^3 of a 0.2mol dm^-3 solution?

Answer 11

1. Find the mols of KOH, by using n=cx(v/1000) - sub in the value of c that has been given in the question;
2. Convert moles into grams, by multiplying this by the molar mass of KOH, this tis the same as the MR, which is the sums of the Ars;
3. This produces a value of g for KOH that the standard solution requires;

Question 12

What is the mass concentration in a titration?

Answer 12

1. The mass concentration of a solute is the mass dissolved in 1dm^3 of solution;
   Units g dm^-3;
2. This is found using c=m/v - if the mass of the solute and the volume of the solvent it is dissolved in are known.

Question 13

Define, and produce examples of dilute and concentrated solutions.

Answer 13

1. Concentrated:
   - Large amount of solute per dm^3;
   - Typically >10mol dm^-3 (in acids);
2. Dilute solutions:
   - Small amount of solute per dm^3;
   - Examples include normal bench solutions of acids - which have concentrations of 1mol dm^-3, or 2mol dm^-3;

Question 14

The difference between concentration of an acid and its strength?

Answer 14

Concentrated acids have a
large amount of solute per dm^3; this has nothing to do with the strength of the acid - the strength of an acid is to do with its H+ ions and how much these are willing to dissociate in solution (where strong acids dissociate almost completely).

Question 15

What does ‘M’ mean? Explain this in terms of what it refers to. Use an example.

Answer 15

1. Molar - this refers to a solution with a concentration in moles per cubic decimetre, mol dm^-3;
2. For example, 2 mol dm^-3 and 2M mean the same thing - 2 mol of solute in 1dm^3 of solution;
3. One should always quote concentrations using the units mol dm^-3;