What is the escape speed for an electron initially at rest on the surface of a sphere with a radius of 1.7 cm and a uniformly distributed charge of 1.8 × 10-15 C? That is, what initial speed must the electron have in order to reach an infinite distance from the sphere and have zero kinetic energy when it gets there? The charge of an electron is 1.602 × 10-19 C and its mass is 9.109 × 10-31 kg.

Explanation:

We know that,

            Energy = .......... (1)

and,          Energy = ............ (2)

Now, equating both equations (1) and (2) as follows.

              =

                       v =

                          =

                          =

                           = m/s

or,                        = 18 km/s

Thus, we can conclude that initial speed the electron must have in order to reach an infinite distance from the sphere and have zero kinetic energy when it gets there is 18 km/s.

first, consult an activity series chart to determine if al is more reactive than fe. it is, so it will replace the fe. now, one can continue with the problem.

next, make sure you have a correctly balanced equation.

3feo + 2al ==> 3fe + al2o3

now, determine which reactant (feo or al) is in limiting supply.

moles feo present = 125 g x 1 mole/71.8 g = 1.74 moles

moles al present = 25.0 g x 1 mole/26.98 g = 0.927 moles

since from the balanced equation, it take 3 mol feo for every 2 mol al, al will be limiting.

you can determine this simply by dividing moles of each by their coefficient in the balanced equation. for example: 1.74/3 = 0.58 and 0.927/2 = 0.464. al is less, so it is limiting.

finally, using the stoichiometry in the balanced eq. determine mass of fe that can be produced from 0.927 moles of al.

0.927 moles al x 3 moles fe/2 moles al x 55.8 g/mole = 77.6 g

it's all four options.