(i)how many atoms are present in 7g of lithium?
(ii)how many atoms are present in 24g of carbon?
calculate the number of moles in each of these substances
19g of fluorine, f2
22g of carbon dioxide, co2
17g of hydroxide, oh- ions

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.

i think that the answer is c

(newtons first law)

conservation of momentum law states :

momentum before collision = momentum after collision

momentum p (kg-m/s)= mass * velocity

say moving car=1 000 kg, velocity = 10 m/s then p = 1 000*10= 10 000 kg-m/s

say still car = 2 000 kg, velocity = 0 m/s then p = 2 000* 0 = 0 kg-m/s

total momentum prior to collision = 10 000 + 0 = 10 000 kg-m/s

momentum after collision = 10 000 kg-m/s

mass now = 1 000 kg+2 000 kg = 3 000 kg

10 000 = 3 000 kg* velocity m/s

10 000/3 000 = velocity

3.33 m/s = velocity after collision

an impulse is a force applied over time , momentum is exclusive of external forces and a perfect collision is implied