You can solve this equation by using the following dilution formula:
`~M_1` x `~V_1` = `~M_2` x `~V_2`
`~M_1` = molarity of the stock solution = 10.0 M
`~V_1` = volume of the stock solution = 20.0 mL
`~M_2` = molarity of the diluted solution = ? M
`~V_2` = volume of the diluted solution = 250. mL
Step 1: Rearrange the dilution formula to solve for `~M_2` .
To rearrange the formula, divide both sides of the formula by `~V_2` . The `~V_2` variables on the right side of the equation cancel out leaving you with:
(`~M_1` x `~V_1` )/`~V_2` = `~M_2`
Step 2: Rewrite the formula so that the unknown variable is on the left.
`~M_2` = (`~M_1` x `~V_1` )/`~V_2`
Step 2: Plug in the known values and solve for `~M_2` .
`~M_2` = [10.0 M x 20.0 mL]/250. mL
= 0.800 M