What volume of water was added to the 350

Question

mL of NaOH solution? Assume volumes are
additive.
Answer in units of mL

Answer 1

c1v1 = c2v2
6M*350 mL = c2*4.2
c2 = 500? Check that.
Then 500-350 = 150 mL added.
Note: That is not exact because volumes are almost never additive. The 500 total volume is correct as is the 350 mL (it was measured) but we assume that 350 mL + 500 mL = 500 mL and that isn't always true.