Andrea’s Help Sheet on Preparing Solutions
There are several types of stock solutions made in the research lab: Percent (%) solutions, Molar (M)
solutions, X solutions, and mg/ml solutions.
First are instructions on how to make % solutions.
First, know the definition of a % solution:
1% = 1g/100ml
That’s the basic formula, and it is logical because “per cent” means “per hundred”.
So, let’s say you are assigned the task of making 1 liter of 20% SDS stock solution, a lab basic.
(On quizzes, you may be told the MW of the chemical; it is not used in % solutions, so ignore it.)
The three steps are:
1: write out the formula:
1% = 1g/100ml
2: scale it up on the percentage:
20%
=
20g
/100ml
3: scale to the final volume by multiplying by 10:
20% =
200g/1000ml
Write out the description in the following manner:
Place 200g of SDS powder a 1L beaker and add H
2
O up to 900ml.
When dissolved, transfer
solution to a 1L graduated cylinder and add H
O to 1000ml.
This works whether for a % w/v solution (as exampled above) or a % v/v solution.
Suppose you
need 100ml of 20% EtOH:
1% = 1ml/100ml
20% = 20ml/100ml
Mix 20ml EtOH with 80ml H
O for a final volume of 100ml.
Next, let’s look at Molar (M) solutions.
First, know the definition of a Molar solution: 1M = 1MW(in grams)/Liter
(MW = molecular wt.)
That’s the basic formula, and is logical because Molar is defined as a solution
concentration having one mole of solute per liter of solution.
So, lets say you are assigned the task of making
100ml
of
5M NaCl
.
Here, you
will
need the
molecular wt for NaCl; it is 58.44.
The three steps are:
1: write out the formula:
1M = 1MW(g)/1000ml
2: plug in the numbers & scale it up on the Molarity:
5
M
=
(58.44)g
/1000ml
& work out the calculations:
5M = 292.2g/1000ml
3: scale to the final volume by dividing by 10:
5M =
29.22g/100ml
Write out the description in the following manner:
Dissolve 29.22g of NaCl in 90ml H
O in a 100ml beaker.
Transfer solution to a 100ml graduated
cylinder and add H
O to 100ml.
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 Spring '09
 LEGGE

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