An oxygen sample has a volume of 4.89 l at 27 °c and 800.0 torr. how many oxygen molecules does it contain?

For solving the question we use the ideal gas law:

pressure × volume = moles × gas constant × temperature

Because you have the pressure in torr the gas constant will have the following value:

gas constant = 62.363 ( L × torr ) / ( K × mol )

To mach the unit of the constant the temperature have to be expressed in kelvin:

T°K = T°C + 273

T°K = 27 + 273 = 300 °K

Now we add the values in the formula:

pressure × volume = moles × gas constant × temperature

800 × 4.89 = moles × 62.363 × 300

moles = (800 × 4.89) / (62.363 × 300)

moles = 0.209

The we construct the following reasoning:

if       1 mole of oxygen contains 6.022 × 10²³ molecules

then 0.209 moles of oxygen contains X molecules

X= (0.209 × 6.022 × 10²³) / 1 = 1.26 × 10²³ oxygen molecules

answer: c. the experiment shows that the red substance experienced a chemical change.

explanation: chemical change is the change in which chemical composition of the substance changes. physical change is the change in which there is no change in chemical composition.

chemical changes are accompanied by following changes:

a) precipitate formation

b) energy is released or absorbed

c) formation of bubbles

d) change in color

as it is given that the red color has disappeared and there is production of gas, it means the red substance has undergone a chemical change.

e

- lead + phosphoric acid = hydrogen + lead(ii) phosphate