Question 498280
In words:
(ml of NaCl you end up with) / (ml of total solution you end up with) = 2%
They are not telling you how many ml of the stock solution you start with.
They only say you end up with 20 ml of solution.
In order to get down from 5% to 2% you must add water ( or whatever the solvent is)
They should have mentioned that
Let {{{a}}} = ml of the 5% solution you need
Let {{{b}}} = ml of water to add
given:
{{{ .05a }}} = NaCl in the 5% solution
(1) {{{ a + b = 20 }}}
---------------
(2) {{{.05a / 20 = .02 }}}
(2) {{{ .05a = .4 }}}
(2) {{{ a = 8 }}}
and, since
(1) {{{ a + b = 20 }}}
(1) {{{ 8 + b = 20 }}}
(1) {{{ b = 12 }}}
You need 8 ml of the 5% solution
and 12 ml of water
check:
(2) {{{.05a / 20 = .02 }}}
(2) {{{.05*8 / 20 = .02 }}}
(2) {{{ .4/20 = .02 }}}
(2) {{{ .4 = .4 }}}
OK