Question

6. A. An isotope with atomic number 84 and mass number 212 emits an alpha particle. what are the atomic number and mass number of the resultant atom?

B. An atom of atomic number 48 with a mass number of 113 emits A beta particle. What are the new atomic and mass numbers ?

7. Why does the pH change when CO₂ dissolves in water _?

            
8. The 32 isotope of phosphorus has a half life of 14.3 days. How long will It take for a sample of 100g to decay to 12.5g of phosphorus?

9. The maximum legal limit of arsenic in water is 0.0500ppm (parts per million) for drinking water. Is water with 8.00x10^-6 moles of As per liter suitable for drinking ? Show how you arrive at your answer.

10. How many grams of NaOH are in 2.00 liters of a 0.255molar solution ?

Answer 1

6.

A. alpha particle is a He nucleus

So the new resultant atom will have,

atomic number = 82

atomic mass = 208

B. beta particle is an electron.

So the resultane atom would have,

atomic number = 47

mass number = 113

7. Dissolved CO2 forms H2CO3

which then dissoicates into H+ and HCO3^2-

the amount of dissociation changes the pH that is H+ concentration in solution and hence the pH of water changes.

8. Using first order kinetics,

ln[A] = ln[Ao] - kt

t1/2 = 0.693/k

k = 0.693/14.3 = 0.0485 d-1

with,

[Ao] = 100 g

[A] = 12.5 g

t = ?

we get,

ln(12.5) = ln(100) - 0.0485 x t

t = 42.87 d

So it would take 42.87 d for the mass reduction.

9. Convert 8 x 10^-6 moles to ppm

8 x 10^-6 x 1000 x 74.9216 = 0.60 ppm

So the amount 8 x 10^-6 moles (0.6 ppm) is way above the allowed limit of As in water and thus not suitable for drinking.

10. molarity = moles/L

moles = g/molar mass

grams of NaOH = 0.255 x 2 x 40 = 20.4 g