# Chemistry Class 12 NCERT Solutions: Chapter 1 The Solid State Part 8

Q: 15. Copper crystallises into a fcc lattice with edge length . Show that the calculated density is in agreement with its measured value of .

Ans:

Edge length,

As the lattice is fcc type, the number of atoms per unit cell,

Atomic mass,

We also know that,

Applying the relation:

The measured value of density is given as . Hence, the calculated density is in agreement with its measured value.

Q: 16. Analysis shows that nickel oxide has the formula . What fractions of nickel exist as ions?

Ans:

The formula of nickel oxide is .

Therefore, the ratio of the number of Ni atoms to the number of O atoms

Now , total charge on

Let the number of is .

Now, total charge on

And, total charge on

Since, the compound is neutral, we can write:

Therefore, number of ions

And, number of ions

Hence, fraction of nickel that exists as

And, fraction of nickel that exists as

Alternatively, fraction of nickel that exists as