18. <span>Answer is </span>
A<span>
<span>Since the enthalpy of reaction is positive, the
forward reaction is<span> an endothermic reaction which means the energy
is gained from the surrounding to happen the reaction. If the temperature
decreases, according to the </span></span>Le Chatelier's principle, the system tries to become equilibrium
by increasing temperature. Since forward reaction is endothermic (because of
the bond breaking), the backward reaction is exothermic (because of the bond
making) which releases the energy to the surroundings. This makes the increase
of temperature. So if the backward reaction is promoted because of the decrease
of temperature, then the concentration of H</span><span>₂ will decrease.</span>
<span>
</span>
19. Answer is A.
The reactant side
has 2 moles/molecules of reactants and the product side has 4 moles/molecules
of products which come from 1 N₂(g) and 3 H₂<span>(g). If the pressure is reduced in the system, according to the Le Chatelier's principle, the
system tries to increase the pressure. </span><span>Hence, forward
reaction is promoted because of the higher number
of molecules in product side. If the forward reaction is promoted, the
concentration of NH</span>₃(g) will decreased.
<span>20. </span>Answer is C.
If the concentration
of reactant is increased in the
system, according to the Le Chatelier's principle, the system tries
to reduce the concentration of that reactant. So if NH₃(g) concentration
is increased, then to be equilibrium, the forward reaction will be promoted.
Then the concentration of N₂<span>(g) will increase.</span>
<span> </span>
<span>A. Exact ecological footprints are often difficult to calculate, but estimates can be useful in comparing populations.
</span>Which of the following could be said about ecological footprints? <u /> <u>Exact ecological footprints are often difficult to calculate, but estimates can be useful in comparing populations.</u><u />
NOT:
b. Ecological footprints can't be used to determine carrying capacity.
C. Ecological footprints don't take into account resources needed to absorb and manage wastes.
<span>D. The average ecological footprints for various countries are nearly identical.</span>
Answer:
C. pH values are different because the 
solution partially ionizes and the HCl solution ionizes 100 percent.
Explanation:
Strong acids are the acids which completely ionizes in the solution while weak acids are the acids which ionizes partially in the solution. They exists in equilibrium in the solution with their respective ions.
Thus, strong acids furnish greater concentration of protons as compared to weak acid when same concentration of both types are taken.
<u>Also, pH is the negative of the logarithm of the hydrogen ions. Thus, hydrogen ion concentration for both acids are different and thus pH will be different.</u>
Clouds are made of water. When water evaporates it turns into clouds. So, I think the answer is EVAPORATED water.
H = planks constant
<span>m = mass of the object </span>
<span>u = velocity of the object </span>
<span>h = 6.626 * 10^-34 J/s </span>
<span>the mass of an electron is 9.12*10^-31 kg </span>
<span>10% speed of light = 10% * 3*10^8 = 3*10^7 m/s, i dont have my graphing calc with me right now so i leave the technicalities up to you </span>
