Answer:
This question is incomplete because of the absence of the molecules been referred to in the question. However, the molecules been referred to in the question is in the attachment below
Explanation:
First of all, we have to define lewis acid and lewis base.
A lewis acid is a substance that has the capacity to accept a pair of electrons. For example, H⁺. While a lewis base is a substance that is capable of donating a pair of electrons (to a lewis acid). For example, OH⁻.
Going by the definitions above, we can deduce the type of substance the molecules in the question (attachment) are
a. BF₃ (boron trifluoride) is a lewis acid because the central atom (as suggested to be focused on in the question) which is <u>boron, has the capacity to accept a pair of electrons</u>.
b. The central atom (carbon) in the compound (2-methyl propane) <u>also has the capacity to accept a pair of electrons</u> since it is positively charged (is electron deficient). Hence, it is a lewis acid.
c. The functional group in this compound (trimethyl borate) is the ether. Ethers are generally lewis bases because the oxygen atom in an ether can donate a pair of electrons from it's lone pair. However, the <u>presence of boron which is central to the compound</u> shows it is also a lewis acid, although weak (majorly due to the presence of the ethers).
The solubility of a given solute in a given solvent typically depends on temperature. Many salts show a large increase in solubility with temperature. ... A few, such as cerium(III) sulfate, become less soluble in water as temperature increases.
Explanation:
A mixture is made of two or more substances that are not chemically combined whereas a compound is made of 2 or more elements that are chemically combined. The elements that make up the compound are combined in fixed ratios.
Answer : The correct option is, (b) 22.5 M
Explanation : Given,
Mass of calcium nitrate tetrahydrate = 266 g
Molar mass of calcium nitrate tetrahydrate = 236.15 g/mole
Volume of solution = 
Molarity : It is defined as the moles of solute present in one liter of solution.
Formula used :

Now put all the given values in this formula, we get:

As calcium nitrate tetrahydrate dissociate to give 1 mole of calcium ion, 2 moles of nitrate ion and 4 moles of water.
The concentration of nitrate ion = 
Thus, the concentration of nitrate ion is, 22.5 M
The standard state formation reaction is a chemical reaction in which one moles of substance in its standard state is formed from its constituent element in their standard state.All the substance must be in their most stable state at 100kpa and 25 degrees celsius.
therefore for HF is
1/2H2 +1/2F2 =HF