Hello!
We have the following information:
Knowing that for each mole of substance we have 6.02 * 10²³ molecules, it is known that in STP (Standard Temperature and Pressure) one mole of any gas equals 22.4 L, then:
22.4 L ----------------- 6.02*10²³ molecules
15.7 L ----------------- y molecules
[tex] \dfrac{22.4}{15.7} = \dfrac{6.02*10^{23}}{y} [/tex]
[tex]22.4*y = 15.7*6.02*10^{23} [/tex]
[tex]22.4y = 9.4514*10^{24}[/tex]
[tex]y = \dfrac{9.4514*10^{24}}{22.4} [/tex]
[tex]y = 4.219375*10^{23}\:\to\:\boxed{\boxed{y \approx 4.22*10^{23}\:molecules}}\end{array}}\qquad\checkmark[/tex]
I hope this helps. =)
