32.417 kg of sodium carbonate.
If you work on US gallons, 30 gallons is 30 x 3.78 = 26.22 litres, and the density is 1.5 kg/l so the mass would be 26.22 x 1.5 = 39.33 kg.
Of this, 98% is nitric acid, so that is 0.98 x 39.33 = 38.54 kg, and molar mass is 63.01 g/mol, so the number of moles is 38,543.4 / 63.01 = 611.70 moles.
The reaction between sodium carbonate and nitric acid would be as follows: Na2CO3 (s) + 2 HNO3 (aq)--> 2 NaNO3 (aq) + H2CO3 (aq)
So if you have 611.70 moles of nitric acid you will need half as many moles of sodium carbonate, which is 305.85 moles. Molar mass of sodium carbonate is 105.99 g/mol.
So you would need: 105.99 x 305.85 = 32,417.04 g, or just under 32.5 kg.
This is if you use waterfree sodium carbonate (white powder). If you use the more usual crystal washing soda,
285.99 x 305.85=87,471 g, or just under 87.5 kg.