HELP with concentration

1. Convert the 5cm³ into dm³.
There are 1000cm³ in 1dm³
5cm³ ÷ 1000cm³/dm³ = 0.005dm³.

2. You find the amount of the substance for which the concentration will change
intiial concentration (mol/dm³) = amount (mol) ÷ initial volume (dm³)
amount = concentration × volume
amount = 1mol/dm³ × 0.005dm³
amount = 0.005mol

3. No more of the substance will be added, but more water will be added, so the amount will stay the same.
final concentration (mol/dm³) = amount (mol) ÷ final volume (dm³)
volume = amount ÷ concentration
volume = 0.005mol ÷ 0.1mol/dm³
volume = 0.05dm³

4. Remember you aren't working out 'final volume', you are working out 'change in volume'
change in volume = final volume - initial volume
change in volume = 0.05dm³ - 0.005dm³
change in volume = 0.045dm³

5. You can convert the 0.045dm³ to cm³ (I don't know if the question needs you to do this)
There are 1000cm³ in 1dm³
0.045dm³ × 1000cm³/dm³ = 45dm³

You would need to add 45cm³ of water

Thanks for the help! It's so much more clear now!

No problem... helped me revise concentrations for myself - it's a win for both of us.
(Thanks for the rep, means a lot to me and for the practise)