for example if slash is 70 power and scores a critical hit what will the power be
Generation I
When a move lands a critical hit, the attacker's level will be doubled during damage calculation, which approximately (but not quite) doubles the damage dealt. [...] For example, a level 5 Pokémon will inflict about 1.5× damage on a critical hit, while a level 20 Pokémon will inflict 1.8× and a level 95 Pokémon will inflict 1.95×.
Other generations
From Generation II to V, a critical hit deals 2 times the normal damage. From Generation VI onward, a critical hit deals 1.5 times the normal damage.
Examples
If the attacker is using a physical move and is at +2 Attack stages while the target is at +1 Defense stage... Gen II: a critical hit will not ignore either stat. (Damage with a critical hit will have an 8/3× modifier.) Gen III-V: a critical hit will ignore the Defense boost. (Damage with a critical hit will have a 4× modifier.) Gen VI+: a critical hit will ignore the Defense boost. (Damage with a critical hit will have a 3× modifier.)
If the attacker is using a physical move and is at -1 Attack stages while the target is at -2 Defense stage... Gen II: a critical hit will not ignore either stat. (Damage with a critical hit will have a 2.64× modifier.) Gen III-V: a critical hit will ignore the Attack drop. (Damage with a critical hit will have a 4× modifier.) Gen VI+: a critical hit will ignore the Attack drop. (Damage with a critical hit will have a 3× modifier.)
If a Pokémon with the Sniper Ability gets a critical hit, the damage dealt will be multiplied by an additional factor of 1.5.
Critical hit (Bulbapedia)
Critical hits hit with double the power, ignoring any defensive boosts. Sniper will triple the effect of critical hits. In Slash's case, it will have 140 base power.
