A pure substance has a constant composition and cannot be separated into simpler substances by physical means. There are two types of pure substances: elements and compounds. Elements: are pure substances made up of only l type of atom. Atoms of the same element are identical in properties.
Answer:
Weak bonds require less energy to form than strong bonds
Explanation:
According to Coulomb's law, the force between two species is inversely proportional to the distance between them. That said, the bigger the atoms are, the greater the bond length should be to form a molecule.
As a result, for a greater bond length, the attraction force is lower than for a shorter bond length. This implies that large atoms would form weak bonds and small atoms would form strong bonds.
Bond energy is defined as the amount of energy required to break the bond. If a bond is weak, it would require a low amount of energy to break it. This is also true for energy of formation, as it's the same process taking place in the opposite direction.
Answer:
27%
Explanation:
Hello,
The following information is missing, but I found it: "1.92 g of sodium sulfate is produced from the reaction of 4.9 g of sulfuric acid and 7.8 g of sodium hydroxide" so the undergoing chemical reaction is:

Now, to compute the percent yield, we must first establish the limiting reagent to subsequently determine the theoretical yield of sodium sulfate because the real (1.92g) is already given, thus, we consider the following procedure:

- The moles of sodium hydroxide that completely react with 0.05 moles of sulfuric acid are:

As this number is higher than the previously computed 0.05 moles of available sulfuric acid, one states that the sulfuric acid is the limiting reagent. Now, the theoretical grams of sodium sulfate are found via:

Finally, the percent yield turns out into:

%
Best regards.
An element with 8 valence electrons is Neon (Ne).
Ps: Any element under group 18 has 8 valence electrons except for Helium (He).