ms = 20 g, c = 35%
=> c% = mdx100/ms
=> md = msxc/100 = 20x35/100
=> md = 7 g HNO3
.............7 g............................. V L
3C + 4HNO3 => 3CO2 + 4NO + 2H2O
...............4x63.........................4x22,4
=> V = 7x4x22,4/4x63
=> V = 2,49 L NO