Afictitious element x is composed of 10.0 percent of the isotope 55x, 20.0 percent of the isotope 56x, and 70.0 percent of the isotope 57x. calculate the weighted atomic mass of element x to the nearest tenth.

56.6

Assuming here that we have 100 atoms of each element. The percentage tells us that we have 10 atoms of the 55 isotope, 20 of the 56, and 70 of the 57.  

So the total is 10*55 + 20*56 + 70*57 = 5,660. And to get the average, we just divide by the total number of atoms (100).  

Atomic mass = 5660/100 = 56.6  

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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