First, let's write the balanced reaction, so we know what we're dealing with

HNO₃ + RbOH  ⇌  H₂O + RbNO₃

Let's apply M =

moles/L

2.0 M RbOH =

x moles RbOH /4 L RbOH

= 8 moles RbOH

Now, we use our reaction equation to do a mole to mole ratio

8 moles RbOH

(1 mole HNO)₃ / (1 mole RbOH)

= 8 moles HNO₃


We know both HNO₃ Molarity and moles, hence we can find L

3.2 M HNO₃

(8 mole HNO₃)/ (L HNO₃)

= 2.5 L HNO₃


To reach the endpoint of this titration, 2.5 L HNO₃ are required