Open spaces such as parks, recreation areas, golf courses, public institutional and public school sites, playgrounds, drainage or other ways which shall be provided at a ratio of not less than 12 acres of open space for every 300 dwelling units. Not less than 25% of the total gross acreage in such open space shall be vacant ground and available to the Township or other public ownership for school sites, parks, drainage ways or other purposes acceptable to the Township Council.