A buffer system usually will need a weak acid, like H2CO3, which is carbonic acid, and a strong base, and the only compound that could figure a fairly strong base among the options given is Na2CO3. NH3 and NaHCO3 are weak bases and HF is a strong acid. Therefore the best answer will be letter A
The answer is (2) low temperature and high pressure so oxygen gas behave least like an ideal gas :)))
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I think the correct answer would be NH4ClO4. It would produce a basic solution since as it dissociates into ions OH ions are being formed as the concentration of this increases, the pH of the solution would increase as well making it basic. Hope this helps.
B. The Secondary side of the step down transformer.