Here's how to calculate the mass of 2.5 moles of sodium hydroxide (NaOH):

1. Find the molar mass of NaOH:

* Sodium (Na): 22.99 g/mol

* Oxygen (O): 16.00 g/mol

* Hydrogen (H): 1.01 g/mol

Molar mass of NaOH = 22.99 + 16.00 + 1.01 = 40.00 g/mol

2. Use the formula:

Mass = Moles x Molar mass

3. Plug in the values:

Mass = 2.5 moles x 40.00 g/mol

Mass = 100 g

Therefore, the mass of 2.5 moles of sodium hydroxide is 100 grams.