# Solution Concentration Which Formula “Which Formula Should I Use?”

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Solution Concentration Which Formula “Which Formula Should I Use?”

Solution Concentrations Used in Lab Explicit: g/L or mg/ml Percent: % Molar: M, mM or µM

Explicit Concentration Expressed as a specific mass per specific volume Mass may be g, mg, µg, etc Volume may be to be L, ml, µl, etc Examples: Examples: X g OR X mg OR Xµg OR Xµg L ml µl ml

Explicit Concentration Tips Formula weight does not matter! Set up as a fraction Convert units as needed to make unit prefixes the same in the numerator and denominator Simplify the fraction until the denominator is equal to 1 Used to re-hydrate lyophilized reagents and make media

Percent Solution X% (w/v) solution = X grams solute 100 ml solution OR X% (v/v) solution = X ml solute 100 ml solution

Percent Solution Tips Formula weight does not matter! Solid solute indicates w /v Liquid solute indicates v /v Gel concentrations expressed this way Look for ” % ”

Molar Solutions g = M FW L g = grams of solute to be weighed out M = molarity of the desired solution FW = formula weight of the solute L = volume of desired solution expressed in liters

Molar Solutions Tips Formula weight does matter Solute is dry Volume must be expressed in liters Usually solving for “g” Add any water(s) of hydration to the FW of the solute Look for “ M ” or “ mM ” or “µM”

Sometimes you need to dilute a stock solution in order to make a working solution… Diluting Solutions

Diluting Concentrated Solutions C s V s = C w V w C s = concentration of stock solution V s = volume of stock solution needed C w = desired concentration of working solution V w = desired volume of working solution

Diluting Solutions Tips Stock buffers diluted this way Solute is liquid Volume usually expressed in ml Usually solving for “V s” Look for “ X ”

Now that you know how to use the formulas, you can practice knowing which formula to use. Continue for practice problems in making solutions in the lab. Read each problem, select which formula you need to apply, and see if you can solve the problem. Practice!

Directions Each practice problem will give you four formula choices. Choose the formula that applies to the problem. Then solve the problem yourself. On each “CORRECT!!” slide, click to see the problem set-up, then click again for the solution so you can check your answer. Then click the blue button to go to the next problem.

Practice #1 You need to make 1L of LB/Amp media. The concentration of Amp in the media should be 0.1 mg/ml. How much Amp powder do you need to weight out? g = M FW L CsVs = CwVw X grams solute 100 ml solution mg/ml choose formula:

Practice #2 We purchase PBS as a 20X stock solution. How would you make 150 ml of 1X PBS? g = M FW L CsVs = CwVw X grams solute 100 ml solution mg/ml choose formula:

Practice #3 Binding Buffer is 4M Ammonium Sulfate [(NH 4 ) 2 SO 4 ]. How would you make 300 ml of this solution? g = M FW L CsVs = CwVw X grams solute 100 ml solution mg/ml choose formula:

Practice #4 A 30 mg vial of Ampicillin is rehydrated with 3 ml of sterile distilled water. What is the concentration of ampicillin in this solution? g = M FW L CsVs = CwVw X grams solute 100 ml solution mg/ml choose formula:

Practice #5 Methylene Blue is used to stain DNA. How would you make 250 ml of a 0.5% solution of Methylene Blue using powdered Methylene Blue? g = M FW L CsVs = CwVw X grams solute 100 ml solution mg/ml choose formula:

Practice #6 Your protocol calls for a 30 ml, 0.8% DNA gel. How much agarose powder do you need? g = M FW L CsVs = CwVw X grams solute 100 ml solution mg/ml choose formula:

Practice #7 You need 10 mM EDTA solution to dilute your DNA. How would you make 50 ml of 10 mM EDTA? g = M FW L CsVs = CwVw X grams solute 100 ml solution mg/ml choose formula:

Practice #8 We have 50X TAE. You need 1X TAE to run your gel. How would you make 400 ml of 1X TAE? g = M FW L CsVs = CwVw X grams solute 100 ml solution mg/ml choose formula:

Practice #9 How would you prepare 250ml of a 40mM solution of Magnesium Chloride [MgCl 2 6H 2 O]? g = M FW L CsVs= CwVw X grams solute 100 ml solution mg/ml choose formula:

Practice #10 We have 2M glucose solution. How would you make 100 ml of 2mM glucose solution? g = M FW L CsVs= CwVw X grams solute 100 ml solution mg/ml choose formula:

CONGRATULATIONS! By carefully reading the problems and picking out the unknown, you were able apply the correct formula and do the math!

SORRY!

CORRECT!! This is a mg/ml problem. You need a concentration of 0.1mg/ml in the media. 0.1 mg/ml is equivalent to 0.1 g/L. Since you are making 1 L of media, you would weigh out 0.1 g of Ampicillin powder and add it to the 1 L of media. Click for next step

SORRY!

CORRECT!! C s V s = C w V w is correct Cs = 20X PBS C w = 1X PBS V w = 150 ml (20X)(V s ) = (1X)(150ml) Cross-multiply & divide: V s = 7.5 ml 20X PBS + diH2O qs 150 ml Click for next step

SORRY!

CORRECT!! This is a molarity problem. g = M FW L FW for (NH 4 ) 2 SO 4 = 132 g 300 ml = 0.3 L g = 4 132 0.3 g = 158.4 of (NH 4 ) 2 SO 4 in di H 2 O qs 300 ml Click for next step

SORRY!

CORRECT!! This is a mg/ml problem. You are adding 3 ml H 2 O to a 30 mg vial so you have 30 mg in 3 ml. Just simplify the fraction: 30 mg = 10 mg 3 ml (1) ml Click for next step

SORRY!

CORRECT!! This is % solution. 0.5 % solution means 0.5 g in 100 ml of solution. You need 250 ml of solution so use this ratio: 0.5 g = X g 100 ml 250 ml X = 1.25 g Methylene Blue + diH 2 O qs 250 ml Click for next step

SORRY!

CORRECT!! This is also % solution. 0.8 % solution means 0.8 g in 100 ml of solution. You only need 30 ml of solution so use this ratio: 0.8 g = X g 100 ml 30 ml X = 0.24 g agarose in 30 ml buffer Click for next step

SORRY!

CORRECT!! This is a molarity problem. g = M FW L FW of EDTA = 372 g 10 mM = 0.01 M 50 ml = 0.05 L g = 0.01 372 0.05 g = 0.558g EDTA + diH 2 0 qs 50 ml Click for next step

SORRY!

CORRECT!! C s V s = C w V w is correct C s = 50X TAE C w = 1X TAE V w = 400 ml (50X)(V s )=(1X)(400ml) Cross-multiply & divide: V s = 8 ml 50X TAE + diH2O qs 400 ml Click for next step

SORRY!

CORRECT!! This is a molarity problem. g = M FW L FW of MgCl 2 ·6H 2 0 = 203 g Don’t forget to add the 6 molecules of H 2 0! 40 mM = 0.04 M 250 ml = 0.25 L g = 0.04 203 0.25 g = 2.03g MgCl 2 ·6H 2 0 + diH 2 0 qs 250 ml Click for next step

SORRY!

CORRECT!! CsVs= CwVw is correct even though we are dealing with molar solutions. First, convert mM to M so units are the same! Cs = 2M glucose Cw = 0.002M glucose Vw = 100 ml (2M)(V s )=(0.002M)(100ml) Cross-multiply & divide: V s = 0.1 ml ml 2M glucose + diH2O qs 100 ml Click for next step