а)
Fe(OH)3 → Fe2O3 + 3H2O
Al2O3 + 3H2SO4 → Al2(SO4)3 + 3H2O
HCl + Cr2O3 → CrCl3 + H2O
4Al + 3O2 → 2Al2O3

b)
2Na + 2H2O → 2NaOH + H2
AlCl3 + 3NaOH → Al(OH)3 + 3NaCl
2Al + 3I2 → 2AlI3
Fe2O3 + 6HNO3 → 2Fe(NO3)3 + 3H2O

Рассчитаем относительные молекулярные массы соединений и восстановим коэффициенты:

а)
Fe(OH)3: 55.85 + (1.01 + 16.00) 3 = 106.92 g/mol
Fe2O3: 55.85 2 + 16.00 3 = 159.70 g/mol
H2O: 2 1.01 + 16.00 = 18.02 g/mol

Fe(OH)3 → Fe2O3 + 3H2O

Al2O3: 26.98 2 + 16.00 3 = 101.96 g/mol
Al2(SO4)3: 26.98 2 + (32.07 + 16.00 4) * 3 = 342.15 g/mol

Al2O3 + 3H2SO4 → Al2(SO4)3 + 3H2O

HCl: 1.01 + 35.45 = 36.46 g/mol
CrCl3: 51.99 + 3 * 35.45 = 158.92 g/mol

2HCl + Cr2O3 → 2CrCl3 + 3H2O

Al: 26.98 g/mol
O2: 16.00 2 = 32.00 g/mol
Al2O3: 26.98 2 + 16.00 * 3 = 101.96 g/mol

4Al + 3O2 → 2Al2O3

b)
Na: 22.99 g/mol
NaOH: 22.99 + 16.00 + 1.01 = 40.00 g/mol
H2: 2 * 1.01 = 2.02 g/mol

2Na + 2H2O → 2NaOH + H2

AlCl3: 26.98 + 3 * 35.45 = 133.33 g/mol
NaCl: 22.99 + 35.45 = 58.44 g/mol

AlCl3 + 3NaOH → Al(OH)3 + 3NaCl

Al: 26.98 g/mol
I2: 2 126.90 = 253.80 g/mol
AlI3: 26.98 + 3 126.90 = 407.68 g/mol

2Al + 3I2 → 2AlI3

Fe2O3: 55.85 2 + 16.00 3 = 159.70 g/mol
HNO3: 1.01 + 14.01 + 3 16.00 = 63.01 g/mol
Fe(NO3)3: 55.85 + 3 (14.01 + 3 * 16.00) = 241.95 g/mol

Fe2O3 + 6HNO3 → 2Fe(NO3)3 + 3H2O