chemical reactions 2?

Question

1.Avogadro's number gives us the number of


A.atoms in any elements or compound
B.particles in a mole of any element or compound
C.atoms in an element
D.particles in a sample



2.The volume in liters at STP for 15 grams of helium is


A.3.75 L
B.56.25 L
C.84 L
D.89.6 L

Answer 1

1.) B. Avogadro's Number allows you to define a mole as a specific number of particles, to allow accurate calculations.

2.) To find out, use the ideal gas law:
PV = nRT

At STP, P = 101.35 kPa, T = 273.15 K, and R = 8.314472 (L · kPa) / (K-1 · mol-1)

n = 15g * 1 mol / 4g He (because He has 2 protons + 2 neutrons)
n = 3.75 mol

Thus:
PV = nRT
V = nRT/P
V = 3.75 mol * 8.314472 (L · kPa) / (K-1 · mol-1) *
273.15 K / 101.325 kPa
V = 84.05 L (B, Solution!)