**Use the dilution equation:**

#C_1V_1=C_2V_2#,

where:

#C_1# is the initial concentration, #V_1# is the initial volume, #C_2# is the final concentration, and #V_2# is the final volume.

**Known**

#C_1="2.00 M"#

#C_2="0.500 M"#

#V_2=200.0"mL"#

**Unknown**

#V_1#

**Solution**

Rearrange the equation to isolate #V_1#. Plug in the known values and solve.

#V_1=(C_2V_2)/C_1#

#V_1=(0.500color(red)cancel(color(black)("M"))xx"200.0 mL"
)/(0.200color(red)cancel(color(black)("M")))= "50.0 mL"# (rounded to three significant figures)

#"50.0 mL"# of the #"2.00 M NaBr"# solution is needed to make #"200.0 mL"# of a #"0.500 M NaBr"# solution.