The mass of KNO₂ is 21 g; the volume of O₂ is 3.0 L.

Part (1) is a problem.

You must make the following conversions:

mass of KNO₃ → moles of KNO₃ → moles of KNO₂ → mass of KNO₂

The balanced equation is

2KNO₃ → 2KNO₂ + O₂

The molar mass of KNO₃ is (39.098 + 14.007 + 3× 15.999) g = 101.102 g

25 g KNO₃ × ##(1″mol KNO₃”)/(101.102″g KNO₃”)## = 0.2473 mol KNO₃

(2 + 2 guard digits).

The molar mass of KNO₂ is (39.098 + 14.007 + 2 × 15.999) g = 85.103 g

0.2473 mol KNO₃ × ##(2″mol KNO₂”)/(2″mol KNO₃”)× (85.103″g KNO₂”)/(1″mol KNO₂”)## = 21 g KNO₂

(2 significant figures)

Part (2) is a problem.

You must convert

moles of KNO₃ → moles of O₂ → volume of O₂

0.2473 mol KNO₃ × ##(1″mol O₂”)/(2″mol KNO₃”)## = 0.1236 mol O₂

Room temperature and pressure are not well defined. I will assume that RTP = 1 atm and 25 °C.

We can use the to calculate the volume of O₂.

##PV = nRT##

##T## = (25 + 273.15) K = 298.15 K

##V = (nRT)/P = (0.1236″mol” × 0.08206″L·atm·K⁻¹mol⁻¹” × 298.15 “K”)/(1″atm”)## = 3.0 L

Hope this helps.