Question 1 Converting units to sensible numbers
Convert the following values to sensible numbers (between 0.1 and 1000) by changing the prefix:
. (i) 0.09 L  
. (ii) 3,000 mM  
. (iii) 0.006 mmol/sec  
. (iv) 6 × 10-5 g/h/kg  

Question 2 Expressing concentration in different units
You have a 0.5 M solution of a compound (molecular weight 200).
a) How many mmoles are there in the following volumes?
. 2 mL  
. 5 mL  
. 10 mL  
. 100 mL  
b) Express this concentration in the following units:
i. mM;  
ii. mg/mL;  
iii. % (w/v);  
iv. mg/dL;  
c) How much of this compound would you have to weigh out if you were to make up 100 mL of the solution?
d) If you added 5 mL of this solution (0.5 M) to a PCR (polymerase chain reaction) with a final volume of 100 mL (2 marks)
(i) How many moles would the reaction contain? (ii) What is the final concentration of this compound in the PCR assay?

Question 3 Difference between #moles and concentration
(2 marks; 0.5 marks each) You have been provided with the following solutions of a compound (mol. wt. = 200). How many mg would be contained in 50 ml?
(i) 300 mM (ii) 10% (w/v) (iii)5 g/dL (iv)1 mg/ml

Question 4 Using molecular weight
You need a 100 mM solution of a potent (and dangerous) transcription inhibitor a-amanitin (mol. wt. 1,250) for an experiment. To avoid handling the powder form of this compound, isolated by a colleague from a deadly mushroom strain, you decide to make up the whole bottle rather than attempt to weigh out a portion. Your colleague gave you 100 mg. What volume would you add to the bottle to make up a 100 mM solution?

Question 5 Using stock solutions in a protocol: from volume to final concentration A DNA digestion with DNase (a pancreatic enzyme that degrades DNA to small fragments) must be stopped by the addition of EDTA. If the protocol instructs you to add 2 mL of stop mix (containing 5 mM EDTA) to an 18 mL digestion reaction what is the final concentration of EDTA in the stopped reaction?

Question 6 Using stock solutions to make up a solution: from final concentration to volume A common reaction carried out in molecular biology is a polymerase chain reaction (PCR). This involves using a DNA polymerase isolated from thermophilic bacteria, Taq polymerase to make multiple copies of a section of DNA. The buffer for this enzyme is made up as a 10 × stock. The final concentration of the buffer in the reaction is: 10 mM Tris, pH 7.5, 50 mM KCl, 1 mM MgCl2. You have the following stock solutions: 1 M Tris, pH 7.5, 2 M KCl and 200 mM MgCl2. How would you make up 50 mL of the 10 × stock buffer?

Question 7 Working with buffers
A citrate buffer must be made up to a final concentration of 60 mM. You need to make up 2 L. You have 2 stock solutions from which to make this buffer; 0.1 M citric acid (C6H8O7) and 0.1 M sodium citrate (C6H5O7Na3). You need to add twice as much of the acid form than the base form to achieve the desired pH. How will you make up this solution?

Question 8 Working with liquids
Z buffer, a buffer used for growing up bacteria for cloning, contains 50 mM b-mercaptoethanol (mol. wt. 78.3), a reducing agent which stabilises many enzymes. b-mercaptoethanol is a liquid with a density of 1.12 g/mL. If you need to make up 200 mls of Z buffer how much b- mercaptoethanol would you need to add?

Question 9 Working with concentrated acids
Phosphoric acid (H3PO4) is supplied as a concentrated liquid. A bottle was assayed and found to have a density of 1.61 g/ml and a purity of 91.3%. The mol. wt. of H3PO4 is 98. What is the molarity of this solution?

Question 10 Putting it all together with common molecular biology buffers
To isolate RNA you often use a protein denaturant solution of guanidinium salts. These chaotropic solutions denature by disrupting H-bonding. The solution also contains a reducing agent to denature disulfide bonds. The final composition of the solution is: 4 M guanidine isothiocyanate (GITC), 25 mM sodium citrate, pH 7.0, 0.5 % (w/v) SDS, 100 mM b-mercaptoethanol. You have guanidine isothiocyanate powder (mol. Wt. 118.16), 1 M NaCitrate, pH 7.0, powder SDS and b- mercaptoethanol (density 1.12 g/mL, mol. Wt. 78.1). How would you make up 100 mL of this solution?

Question 1 Converting units to sensible numbers Convert the follo...