54. According to Sample Problem 41-6, N
x
/N
0
= 1.3
×10
−38
. Let the number of molesof the lasing material
needed be n; then N
0
= nN
A
, where N
A
is the Avogadro constant. Also N
x
= 10. We solve for n:
n =
N
x
(1.3
× 10
−38
) N
A
=
10
(1.3
× 10
−38
)(6.02
× 10
23
)
= 1.3
× 10
15
mol .