Determine the composition of the following cobalt alloys. a. One-third of the Co

Question

Determine the composition of the following cobalt alloys.

a. One-third of the Co atoms are replaced Zn atoms.

b. One-eighth of the Co atoms are replaced by Ti atoms.

c. One-third of the Co atoms are replaced by Zn atoms and one-sixth of the Co atoms

       are replaced by Ti atoms.

Enter your answers numerically separated by commas.

You think this is simple and a. would be 33.33%, 66.67% but the computer says this is wrong. I even tried rounding but still wrong

Same with b. I put 19.7%, 80.3% and it is saying it is wrong.

Chegg even gives these answers but mastering chemistry says they are wrong.

Explanation / Answer

Solution :-

a. One-third of the Co atoms are replaced Zn atoms.

Solution :- assume we have 1 mole of the alloy

Then it says 1/3 of the Co atoms are replaced by Zn

So 1/3 = 0.333

This means moles of Zn = 0.333 and moles of Co = 1-0.333 = 0.667

Now lets calculate the mass of the each

Mass of Co = moles * molar mass

                     = 0.667 mol * 58.933195 g per mol

                         = 39.308 g.

Mass of Zn = 0.333 mol * 65.38 g per mol = 21.7715 g Zn

Total mass of alloy = 39.308 g + 21.7715 g = 61.0795 g

Now lets calculate the percent of each element

% Co = (mass of Co/ mass of alloy)*100%

           = (39.308 g / 61.0795 g)*100%                              

           = 64.36 % Co

% Zn = 100 % - 64.36 % = 35.64 %

b. One-eighth of the Co atoms are replaced by Ti atoms.

Solution :- 1/8 = 0.125

So we have 0.125 mol Ti and 0.875 mol Co

Now lets calculate the mass of alloy mole

Mass of Ti = 0.125 mol * 47.867 g per mol = 5.9834 g

Mass of Co = 0.875 mol * 58.933195 g per mol = 51.566 g Co

Mass of alloy = 51.566 g + 5.9834 g = 57.5494 g

% of Ti = (5.9834 g / 57.5494 g)*100%

              = 10.40 % Ti

% of Co = 100 % - 10.40 % = 89.60 %

c. One-third of the Co atoms are replaced by Zn atoms and one-sixth of the Co atoms

       are replaced by Ti atoms.

Solution :- 1/3=0.333 mol Zn

1/6 = 0.166 mol Ti

Moles of Co = 1 – (0.333+0.166) = 0.501 mol

Mass of each element is as follows

Mass of Zn = 0.333 mol * 65.38 g per mol = 21.7715 g

Mass of Ti = 0.166 mol * 47.867 g per mol= 7.9459 g

Mass of Co = 0.501 mol * 58.933195 g per mol = 29.5255 g

Total mass = 21.7715 g + 7.9459 g + 29.5255 g = 59.2429 g

% Co = (29.5255 g / 59.2429 g)*100%   = 49.84 %

% Zn = (21.7715 g/59.2429 g)*100% = 36.75 %

% Ti = 100 % - (49.84 % + 36.75%) = 13.41 %

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