Answer: K and Na
Explanation:
Potassium and Sodium have a chemical symbol of K and Na respectively.
They have similar chemical properties because both
- have only one valence electron,
- form univalent positive ion when they donate their lone outermost electron as shown below
Na --> Na+ + e-
K --> K+ + e-
- are good reducing agents
- react with cold water vigorously to liberate hydrogen gas and form alkalis, so they are known as alkali metals.
- K and Na are placed in Group 1 of the periodic table.
Thus, unlike the other pairs given, K and Na, have the most similar chemical properties
Answer:

Explanation:
Hello,
In this case, the enthalpy of combustion is understood as the energy released when one mole of fuel, in this case octene, is burned in the presence of oxygen and is computed with the enthalpies of formation of the fuel, carbon dioxide and water as shown below (oxygen is circumvented as it is a pure element):

Thus, since we already know the enthalpy of combustion of the fuel, for carbon and water we have -393.5 and -241.8 kJ/mol respectively, thereby, the enthalpy of combustion turns out:

Best regards.
I'm pretty sure that significant figures is just the amount of numbers there is. So, in this case I think the answer would be D. 5