Equal volumes of h2 and o2 are placed in a balloon and then ignited. assuming that the reaction goes to completion, which gas will remain in excess?
2h2 + o2 → 2h2o

The reaction is

2H₂(g)  + O₂(g) ---> 2H₂O

Thus as per balanced equation two moles of hydrogen will react with one moles of oxygen.

There is a directly relation between moles and volume. [One mole of each gas occupies 22.4 L of volume at STP]

Thus we can say that two unit volume of hydrogen will react with one unit volume of oxygen

Now as we have started with equal units of volume of both oxygen and hydrogen, half of oxygen will be consumed against complete volume of hydrogen

so the gas which will remain in excess is oxygen

you need to take a look at the periodic system in order to sort these elements into pairs that would most likely exhibit similar chemical properties. first, you need to find elements that are found in the same group (groups are vertical in the periodic system), and then to see whether they have the same valence number. if they meet these criteria, they will have similar properties.

the first pair: ne (neon) and kr (krypton), are noble gases found in group 8

the second pair: li (lithium) and k (potassium), found in group 1

the third pair: s (sulfur) and te (tellurium), found in group 6

the answer is option a.