Paul is studying a substance that is made out of only one element. this means that
a.
all the atoms in the substance are similar to each other.
the substance is too small to be seen with the human eye.
the substance does not have any energy
od.
there are many different types of atoms in the substance

A. All the atoms in the substance are similar to each other.  

Explanation:

All the atoms in an element are similar to each other.

B. is probably wrong. Paul could be a brilliant student who can study substances on a scale that can be seen only under a microscope.

C. is wrong. All substances have energy.

D. is wrong. If there are many different types of atoms, the substance is a compound or a mixture.

the ratio of the conjugate base to the acid required to attain the desired ph.

as given the equation is

2al + 6 hcl > 2 alcl3 + 3 h2(g)

as per equation two moles of aluminum reacts with six moles of hcl to give two moles of alcl3 and three moles of hydrogen gas

given: three moles of al are present and 7 moles of hcl

for three moles of aluminum nine moles of hcl will be required

however as 7 moles of hcl are present so the limiting reagent is hcl

hcl is going to decide the number of moles of product formed

when six moles of hcl are present = 3 moles of h2 are formed

when one mole of hcl is present == 3/6 mole sof h2 will be formed

when 7 moles of hcl are present = 3 x 7/ 6 moles of h2 will be formed = 3.5 moles of h2