A balloon contains 12.1 L of helium gas at STP. What is the mass in grams of He in the balloon? O 2.16 g He
O 0.545 g He
O 1.09 g He
O 4.32 g He

The atomic number of cesium = 55

So number of electron in cesium = 55

While number of protons in cesium = 55

Atomic mass of cesium= number of protons + number of neutrons  

Number of neutrons in cesium is given as 78

Atomic mass of cesium= number of protons + number of neutrons  

Atomic mass of cesium=55 + 78

                                       = 133

So, atomic weight of cesium is 133