The examples in the question are neither in Genitive case, nor in Accusative case.  They are in a lesser know *Partitive case.*  Please see this excellent answer: [What are the lesser known Russian cases?](https://russian.stackexchange.com/a/405/4903)

Sometimes nouns in Partitive case coincide with their forms in Genitive case, but sometimes they have their own form.

Here are some examples:

> **Чай / tea** —  Genitive:  *Нет чая;* Accusative: *Я люблю чай;* Partitive: *Налейте стакан чаю.*

> **Сахар / sugar** —  Genitive:  *Нет сахара;* Accusative: *Я люблю сахар;* Partitive: *Насыпьте ложку сахару.*

> **Воздух / air** —  Genitive:  *Без воздуха;* Accusative: *Мне нужен воздух;* Partitive: *Хочу хоть глоток воздуха.*

> **Вода/ water** —  Genitive:  *Из воды;* Accusative: *Включите воду;* Partitive: *Плесните немного воды.*

> **Горчица / mustard** —  Genitive:  *Нет горчицы;* Accusative: *Купите горчицу;* Partitive: *Возьмите ложку горчицы.*

In such examples where in English one would say: "I want *some water/some tea/some sugar/some peace/etc.*", in Russian one would use Partitive case.