Question

What's the protocol for making 200mL of 15% (w/v) aqueous solution? Thanks a bunch in advance!

Answer 1

Dissolve `"30 g solute"` in a small amount of water, then add water to `"200 mL"`.

Explanation:

Determine the mass of solute required to make `"200 mL"` of a `"15%w/v"` aqueous solution.

`"%w/v="("mass of solute in grams")/("volume of solution in mL")xx100%`

Plug in known values.

`"15%w/v="("mass of solute in grams")/("200 mL")xx100%`

Divide both sides by `100%`.

`(15color(red)cancel(color(black)("%"))"w/v")/(100color(red)cancel(color(black)("%")))=("mass solute")/("200 mL")xxcolor(red)cancel(color(black)(100%))/color(red)cancel(color(black)(100%))`

Simplify.

`"0.15 w/v"=("mass solute")/("200 mL")`

Multiply both sides by `"200 mL"`.

`"200 mL"xx"0.15 w/v"="mass solute"`

`"30 g"` `=` `"mass solute"`

Answer 2

Multiply the percentage by the volume to get the necessary mass of solute.

Explanation:

When making a solution of a specified concentration. The units of the concentration are very important. Normally, the percentages labeled on day-to-day grocery products such as vinegar or rubbing alcohol are in mass (in grams) over volume (in milliliters). Concentrations, in general, can be expressed in many different ways making the organization and consistency of the used units very important. Assuming the concentration of your solution is in grams over milliliters, the necessary grams of solute would be 30 grams.

`(grams)/(milliliters)=concentration`
`grams=(concentration)*(milliliters)`

The basic formula for concentrations is as follows and can be manipulated to calculate needed quantities.

`"units of solute"/"units of solution"=(concentration)`