Question

Question #0e523

Answer 1

`9.56"g"`

Explanation:

You have quoted molar heat capacities from your data book but you have used `m` for mass in your expression for `Q`.

So we must work in moles and then convert back to grams using the idea that no. moles = mass/mass of 1 mole

Or in symbols:

`n=m/M_r`

So:

`Q=nxxC_vxxDeltaT`

Note that `n` is the no. moles.

Applying this first to helium:

The `M_r` of helium is 4. (Same as the `A_r`).

So from our expression for `n` we can write:

`Q=(2)/(4)xx12.5xxDeltaT " "color(red)((1))`

Now for oxygen:

The `M_r` of `O_2` is 32 so:

`Q=(m)/(32)xx20.9xxDeltaT " "color(red)((2))`

Now we can put `color(red)((1))` equal to `color(red)((2))rArr`

`2/4xx12.5xxcancel(DeltaT)=m/32xx20.9xxcancel(DeltaT)`

`:.m=(0.5xx12.5xx32)/(20.9)=9.56"g"`

(I have used approximate `M_r` values. You should use the ones you are given)