Calculate how to make 10 ml of a 7.5% methylene blue stock solution (from solid

ID: 477491 • Letter: C

Question

Calculate how to make 10 ml of a 7.5% methylene blue stock solution (from solid powder). Calculate how to make a series of 1:10 dilutions of this stock. Imagine you'll use 5 ml of each dilution for your experiment. Calculate the final concentrations of each tube. take some stock, dilute it 1:10 with some water in a new tube = #1 take some of #1, dilute it 1:10 with some water in a new tube take some of #2, dilute it 1:10 with some water in a new tube take some of #3, dilute it 1:10 with some water in a new tube First, decide the total volume to make of each tube - keep in mind measurement accuracy (consider preparing 20% more solution than what you will use). Then, calculate how mu Ch stock, water and final concentration of each. Then, instead of doing a serial dilution, calculate making each of the above solutions using the weight/volume % solution method. Pros and Cons of serial dilution method vs W/V % method

Explanation / Answer

Ans. Part A: Assume, the provided sample of methylene blue crystals is pure (100%).

Preparation 10.0 mL of 7.5% methylene blue:

Volume of stock solution = 10.0 mL

A 7.5 % (w/v) means 7.5 g solute in 100.00 mL solution.

So, amount of solute required for 10.0 mL solution = (7.5 g/100.0 mL) x 10.0 mL

= 0.75 g

Preparation: Accurately weight 0.75 g methylene blue crystals and transfer it a small amount of water taken in 10.0 mL standard volumetric flask. Make the final volume upto mark (10.0 mL) with distilled water. It is 10.0 mL of 7.5 % methylene blue solution.

Concertation of stock solution = 0.75 g/ 10.0 mL = 0.075 g/mL

Part C: Before going for part B, first calculate the volume of aliquots required. It is mentioned that-

I. 5.0 mL of each dilution is required

II. Prepare the dilution in 20% excess. That is, 5.0 mL + (20% of 5.0 mL) = 6.0 mL. In other words, we need to prepare around 6.0 mL of each dilution.

Part B: Prepare 1:10 serial dilution: 1:10 serial dilution means that there is 1 part of stock solution in a total of 10 parts of the solution. Alternatively-

Dilution ration = volume of stock solution/ volume of final solution = 1/10 = 1:10.

1st 1:10 Dilution: Note that we have to prepare only 6.0 mL of final solution; NOT 10.0 mL.

So,

10.0 mL of final solution contains 1.0 mL stock solution

Or, 1.0 mL - - - (1.0/10.0) mL

Or, 6.0 mL - - - (1.0/10.0) x 6.0 mL = 0.6 mL

Therefore, 6.0 mL of final solution contains 0.6 mL of stock solution.

Re-check the dilution ratio: Dilution ratio = volume of stock solution/ volume of final solution

= 0.6 mL / 6.0 mL = 1/10 = 1:10.

“we are doing correct”.

Preparation of first dilution: Transfer 0.6 mL of stock solution to a clean and dry tube 1. Add 5.4 mL note of distilled water to make the volume upto 6.0 mL. It is the first dilution.

Note: volume of water required = final volume – volume of aliquot = 6.0 – 0.6 mL = 5.4 mL.

2nd serial Dilution: Transfer 0.6 mL from tube 1 to tube 2. Add 5.4 mL distilled water to it to make the final volume upto 6.0 mL. The current dilution ration or factor for tube 2 is also 1:10.

Note: 1. Total dilution till tube 2 = current dilution factor x previous dilution factor

= (1:10) x (1:10) = 1:100

2. Concentration of tube 2 = conc. of stock solution x total dilution factor upto tube 2

= 0.075 g/mL x (1:100)

= 0.00075 g/mL

Similarly prepare serial dilution for tube 3 and 4.

Part D calculations can be done using note 1 and 2 of “2nd serial dilution”.

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Calculate how to make 10 ml of a 7.5% methylene blue stock solution (from solid

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Calculate how to make 10 ml of a 7.5% methylene blue stock solution (from solid

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