Following are a bunch of questions dealing with the making of solutions. They are grouped as to type, and within each group they are listed with the harder ones lower in the list.

Notes to the teacher are in purple. The purple sections are separated from the black for ease in cutting and pasting.

1. Solids or pure liquids dissolved in water
    
   1. Pecent solutions
       
      1. You are told to make a liter of a 1% aqueous solution of NaCl. How much solid NaCl (molec wt = 58) should you use?

         (Should be able to be done in the head. Of course the molec. wt of NaCl is not needed, but should always be included in problems because in real life it is a number that sits there on the bottle to be read. Students need to sort through all the facts in reality and gleen those that they need.)
          

      2. You are told to make a quart of a 1% aqueous solution of NaCl. How much solid NaCl (molec wt = 58) should you use?

         (This will require either converting the quart to metric, or using ounces of NaCl.)
          

      3. You are told to make a liter of a 0.9% aqueous solution of NaCl. How much solid NaCl (molec wt = 58) should you use?

         (Also should be able to be done in the head. But it would be different if something like 893 ml was wanted.)
          

      4. You are told to make a 2.5% agar solution for a certain type of petri plate experiment. How much agar powder should you use to make 200 ml?

         (In the head!)
          

      5. You are told to make a 2.0% agar solution for 10 petri plates of L-broth+agar ("L-agar"). How much agar powder should you use to make 300 ml?

         (In the head!)
          

      6. You are told to make 100 ml of a 1.0% agar solution in L-broth for slants to store your bacterial collection. How much agar powder should you use to do this?

         (In the head!)
          

      7. The major shampoo detergent "cetavlon" is more lethal to bacteria than is the antibiotic streptomycin. You need to determine the minimum lethal concentration on E. coli. You have been told it is about 0.0002%. You have a bottle containing about 50 gm of pure cetavlon. How should you make a tube containing 1 ml of 0.0003% cetavlon? (Due to a severe drought, you have only 90 ml of water to use.)

         (Two hurdles here: the first is how to make serial dilutions, and the second is the limitation on water. One could add water to 1 gm of CET up to 10 ml (10%), and from there move 1 ml into 9 mls of water serially for 1%, 0.1%, 0.01% and 0.001%. From that last 3 ml would be added to 7 ml water for 0.0003%.)
          

      8. You want to test various compounds for their osmotic consequences on the growth rate of brewer's/baker's yeast (Saccharomyces cereviseae). You want to use NaCl (molec wt = 58) at 1% and glycerol at 5%. How much glycerol (molecular weight = 92) should you use to make 500 ml of the appropriate growth medium?

         (Most students with only a little practice can do this in their heads.)
          

   2. Molar solutions
       
      1. You are told to make a liter of a 1M aqueous solution of NaCl. How much solid NaCl (molec wt = 58) should you use?

         (In the head!)
          

      2. You are told to make a liter of a 0.15M aqueous solution of NaCl. How much solid NaCl (molec wt = 58) should you use?
          
      3. You are told to make a quart of a 0.15M aqueous solution of NaCl. How much solid NaCl (molec wt = 58)should you use? (Due to a draught, you have just 980 ml of water available to you.)

         (Here conversion of the quart to metric is necessary.)
          

      4. You want to test various compounds for their osmotic consequences on the growth rate of brewer's/baker's yeast (Saccharomyces cereviseae). You want to use NaCl (molec wt = 58) and glycerol at 0.1M. How much glycerol (molecular weight = 92) should you use to make 500 ml of the appropriate growth medium?
          
2. Solutions dissolved in water to make lower concentration solutions Most of these are straight CV=CV problems.
    
   1. On the side-shelf you see what appears to be about a liter of a 10% aqueous solution of glycerol (molecular weight = 92). How much of it should you use to make 500 ml of a 1% glycerol solution?

      (This and the next three questions are a graded series from simple "in-the-head" questions to harder ones - but the pattern of solving them should readily emerge.)
       

   2. On the side-shelf you see what appears to be about a liter of a 5% aqueous solution of glycerol (molecular weight = 92). How much of it should you use to make 100 ml of a 2% glycerol solution?
       
   3. On the side-shelf you see what appears to be about a liter of a 2 M aqueous solution of glycerol (molecular weight = 92). How much of it should you use to make 250 ml of a 1 M glycerol solution?
       
   4. On the side-shelf you see what appears to be about a liter of a 2 M aqueous solution of glycerol (molecular weight = 92). How much of it should you use to make 250 ml of a 0.3 M glycerol solution?
       
   5. On the side-shelf you see what appears to be about a liter of a 5% aqueous solution of NaCl (molecular weight = 58). How much of it should you use to make 300 ml of a 2% aqueous NaCl solution?
       
   6. Normal Saline ("NS") is 0.15M aqueous NaCl. You have found two bottles on the self. One contains solid NaCl, the other contains 0.5% aqueous NaCl. How much of which should you use to make a liter of NS?
       
   7. The major shampoo detergent "sodium dodecylsulfate" (SDS) has been found to totally prevent the formation of bacterial LPS layers at a concentration of 0.1%. You, however want to see if SDS shows partial interference at lower concentrations. You decide to make L-broth tubes containing these concentrations: 0.0%, 0.001%, 0.01%, and 0.1%. You have found a bottle of 10% SDS among the reagents used to run SDS-PAGE electrophoresis. Describe how you should make the four tubes that you desire.

      (Serial dilution needs to pop into mind.)
       

   8. You are to work with the powerful antibiotic streptomycin. You need to determine the minimum lethal concentration on E. coli. You have been told it is about 0.0003%. You have a 100 ml of 3% streptomycin. How should you make a tube containing 1 ml of 0.0003% streptomycin?

      (Serial dilutions are the most precise and least wasteful way to do this. Remember that trying precisely to weigh anything less than 0.01 gram is not good.)
       

3. Tricky questions If the students really know the stuff above, these aren't all that bad. Plus the experience with having to make choices is good.
    
   1. On the side-shelf you see two full liter containers labelled NaCl (molec wt = 58): one contains dry granules, and the other contains 1.5% (aqueous sol'n). How much of which should you use to make 250 ml of a 2% solution?

      (The trick here is that one cannot dilute from a more dilute solution to arrive at a more concentrated one; the math works, but it doesn't work in reality. Of course one could boil away some of the water, but time is money, and it would be much quicker to dissolve some solid NaCl in water than to boil down the dilute solution to make it more concentrated.)
       

   2. Mary Lou needs to make 3 liters of L-broth containing agar. This calls for 0.3% glucose, 0.7% tryptone, a trace of thiamine, and 2% agar. You have weighed out 9 gms of glucose, 21 gms of tryptone, a smidgeon of thiamine, and 60 gms of pure agar. These, she dumped into a 4 liter flask, with the intention of dissolving it all in 3 liters of tap water, and then distributing it among 6 smaller flasks for easier handling while pouring plates. These 6 flasks would be capped and autoclaved. What problem is Mary Lou going to have?

      (Agar does not dissolve in cool or warm solutions, so that when ML distributes her concoction the agar settles out. The first flasks contain little agar, and plates made from them won't solidify, while plates made from the last flask will set like concrete!)